We used this as our crib. https://www.guavafamily.com/products/lotus-bassinet?_gl=1*1bj64ce*_up*MQ.._gsMQ..&gclid=CjwKCAjwsrbTBhAvEiwA0Bpp4SHu7plwubJ7TphXpjMM1M8nc_gT6tnACnZ1uExV5cEJPSyTXGKfAxoC7CcQAvD_BwE&gbraid=0AAAAADs5YJ42iYke-NcnjYAHz45CwGRUP&_su_rec=QQRuJHTLHtn0wBOzlLKcuEWZPZNEOXAzAYw3w0qJ03yR0JWGYsyQZRsGj-7MlvhHqv6aN0wexD_lbLVhEdom9RTzcDmjaHJyYJdRjrfvXyFWyEyZzuX6uHf9TIzoTa0JWioB2GhE-YoCbgovqWpwxoqpDLYYtUFv3KhKGLpyPVmyJ0X1MTHs9QJI1u_pDiEDEUMNF0zbIgRbau3l-L3stYCSEEWOuDl2CIYxs25E8UXsjyDYrWQ7dfNAeFO7WMT4JHVeemJBEGbxd8Kvc__bLWEmjccxlVRb9D6Ej5lMQVXqV9PGgbbTNqsXnZzgELpRRxkbf9mUfaTNjhQLpoDoLABS6HvWOKVJBhyaky9bgiYUzL0sJLCj-mBzWhDmJHPpEc6sEgkI2ldp&_su_rec_id=e4e5dc00-6f52-4b4e-81be-26f70ecdbd0e-1785647099&variant=470467017
It converts from bassinet to regular size. When not in use it is easy to store. My locals would borrow it when they traveled. Their friends would borrow it when they traveled.
I bought mine on a great Black Friday sale. I’m sure you could find a second hand one.
I bought a stroller used. I eventually bought my own car seat as once they are out of the infant seat they are hard to move from car to car. I pick up two one a week from preschool so I now have two seats in my car. A changing pad on top of a dresser is nice to have.
It is a very nice portable crib but not a full size one, so a baby will likely outgrow it by the time the baby is one year old.
Our kids seriously considered that Guava for their travel abroad but ended up with a Bugaboo.
I am “less is more” when it comes to baby stuff as grandparents. I would wait until they are ready to come for a visit. A newborn’s needs are very different than a 6 month old.
D1 started traveling with her kids since they were 10 weeks old. They were able to bring everything they needed for the kids - stroller, car seat, crib, toys, diapers, formula,etc. And they flew.
Now GD2 is potty trained, they bring a portable patty seat when they go out.
Babies grow up fast, no need to have too many gadgets, especially duplicates (a lot of plastics).
I have a pack and play and a high chair at my house. I also bought this booster seat ( picture). I would recommend the booster seat instead of a high chair as it takes up very little space and can be moved easily from the kitchen table to the table on the screened porch.
Definitely ask in your Buy Nothing ( or other) groups. I got 2 baby walkers from mine ( GS is using them currently). Toys and books are easily available from BN also.
I am jealous of those of you with good Buy Nothing groups. Ours are not filled with treasures, unfortunately.
The BN groups vary widely. The one I did in NoVA was ok, not great but the one here in my current neighborhood is very active and there are a lot of nice items on it
I like this idea of the high chair! When here at our house we tend to eat all over the place - inside, outside, sun porch, etc. That also looks good for that 6-12 month period - can start to sit up straight but not too big.
Weirdly we don’t have a Buy Nothing group for our mid-sized city that is near me! We are literally 2 blocks from a suburb though so I “applied” last night - we will see how picky they are!
Sounds like most necessary items to consider for baby #1 who lives 2.5 hours away and won’t be here on a regular basis but frequent throughout the year is:
Something to sleep in
Something to feed in
Sleep aid (monitor - parents could bring their’s but an easy thing to get and store)
Second hand stroller - again, parents can bring but if I get one good for the duration of time a child needs a stroller probably will come in handy.
Basics: blankets, play mat, etc. We already have a library of books here. 
I think this is a great list. Less is more! Our kids would always bring their strollers so we did not buy any. We eventually bought a foldable wagon which we used to take big grands to the playground and now use for other purposes as well. Came in handy for setting up a party in the park recently.
I recommend getting a basic (not wifi) baby monitor because even though it’s a portable device, it is a PITA to move the camera if it has been installed somewhat permanently. I still use ours!
And a wagon or nothing might be the right choice! I’ll admit, a stroller is something I most would look forward to getting/having! Taking little ones for a walk is a something I adore doing and to be honest, strollers today are so colorful and cute without having characters and such on it when our kids were little. Give me a solid red or orange or blue stroller and a baby and off we go! 
The inexpensive “umbrella “ strollers are still sold if you don’t find a fee one.
A friend in NoVA who is not in a BN group scored a lot of stuff ( crib, stroller, high chair) in her community Facebook group.
I second this suggestion. I bought a really inexpensive, basic monitor. It worked just fine.
This is the one we have:
Just used it last night - Little Miss Mess had a sleepover at our place. We never used the two way talk but temperature sensor is a useful feature.
Once they outgrow the bassinet feature it converts to a larger crib. Our grandkids used it till age 3. We now use a blow up toddler sized bed that my daughter leaves at my house.
My kids use Facebook Marketplace to buy and sell. The locals also use a local parent group community to buy, sell and borrow. The group also has boards for travel advice and any sort of parenting questions parents have. A local mom started the group during Covid.
I bought an inexpensive Graco highchair that we have used a lot. The drawback was that the angle of the back of the chair isn’t as upright as some young eaters need.
GD2 uses this at home. Stokke Tripp Trapp Toddler High Chair: Adjustable High Chair | Stokke® Online Shop
D1 brings this to my place or when they are outdoor, Portable Booster High Chair – Foldable Aluminum Seat with Tray & Travel Bag - Babies Mall
I second having a booster seat (we have hauled out the one that DS3, now a rising college sophomore, used, and it’s not in great shape). And some kind of blocks or toys to play with, maybe some board books. And baby shampoo.
Regarding the “other” grandparents babysitting for weddings - in my family they are generally invited and then whisk the baby away as needed. In fact, my kids’ paternal grandparents helped like this at both of my sisters’ weddings where they were young, and then my parents helped at my BIL’s wedding. And at my son’s wedding, my other son’s inlaws were there just last weekend (!) to help with GS I think babies/children really enhance weddings, though I came to this decision well after my own wedding.
Regarding the 3-week-old, of COURSE it was never put down!! It’s practically a newborn.
We have had a couple of chats with GS while we have been on our trip. Since we are hiking all day the post daycare call has come just before or after we have gone to sleep. Its still been fun to chat.
As noted above, we have had fun sending little 1 minute videos to him about our day.
Yesterday we got a video of him playing piano and singing a song he just made up ““We’re going to daddy’s work it’s so beautiful and far and we’re going soon.” My SIL works at a historic site and D and GS were headed there yesterday
My daughter was in the hospital for 3 months, so was used to being in a crib. She loved sleeping flat (so that’s what I know; she didn’t really like to be held).
The bigger thing was that these parent rarely let anyone hold him (3 week old) so didn’t want him at the wedding with all those people! I have not seen him or held him yet. He did make a quick appearance for family photos (probably before the wedding) and was then whisked away by the grandmother. The 8 month old is a very fussy baby so not really ‘wedding safe’. The 11 month old is adorable and chill. He might have stayed at the wedding longer. Plus all the moms (sisters of the bride) were in the wedding so couldn’t help with care.
Re baby equipment - I was at the beach 2 weeks ago and very much enjoyed watching parents set up all kinds of baby equipment, especially the little tents, lean-to, tarps. One couple right in front of me had this tarp thing and were having a lot of trouble setting it up with 4 poles and they kept putting more sand in the little pockets that stretch it. The wife got fed up and took the little guy to the water. Husband got one side up and realized it worked better as a lean to with just 2 poles. They tried to put the boy down for a nap in the wagon. No go! so the father got him to sleep but had to hold him for the nap. Then all 4 grandparents arrive and decided the tarp thing needed to be supported by all 4 poles and they all started redoing it - while the father was holding the boy in his arms in a beach chair under this tarp that kept falling on him. It was quite comical. The wife stayed out of it and let the grandparents do whatever they wanted.
So the point is that babies need shade but there were all kinds of solutions, from a little baby tent to a full camping tent to the shade tarps. And please be the grandparents who show up late and redo it all. I needed the people watching entertainment!
I am just back from a week at a lake with my kids plus partners plus grandkids. This is our third time doing this trip and I’m hoping we do it again next year. It was great having the cousins all together. The older two age 4 and 3 1/2 had a great time bonding. They are at the age where they played together versus playing side by side.
It’s interesting watching the different parenting styles. One night one of my kids let her toddler skip a bath. Her cousin was fascinated that they were skipping bath time. My other two wouldn’t even consider no bath after a day in the lake and spa. Snacking,sugar, food choices for meals and TV were other areas that differed. It all worked.
My kids parent on two opposite sides of the spectrum, but have learned to deal each others styles. My son is very lenient, my daughter very type A, yet the children all seem to handle it very well. It took my daughter and her husband time to understand how my son and his wife could parent the way they do.
I do wish they could all move just a bit more center, especially my son, by not my circus! It only becomes a problem when I have his kids alone as they like to push the limits, but they also know I am not going to give in. They are well adjusted kids and sweet, so I will keep them! With my daughter’s daughter, she does what is asked of her; while not perfect, she is pretty darn close!