They are out of vogue perhaps but I liked the sewing cards that are threaded with a shoelace. A coordination lesson tha is best learned with an adult at hand. Books that are in line with things you care about and a lap to read them from.
We have a Brio train set in our attic…same reason as your Thomas set.
Your preschooler can help you make homemade sewing cards. Print off or tear out their favorite characters from coloring books, color if you like ( 
), then glue on cardboard, punch the edges with a paper puncher and then take yarn, wrap a little tape around one end to make it easier to “needle” through and voila, homemade sewing cards.
The sewing part and the unsewing part of this task are both good motor skills for young children. Good for traveling or when you need a quiet or play-in-place activity.
This is what we have currently. Age appropriateness is important, as we have grandchildren from baby to an 8 year old.
A bookshelf with books from Indestructibles books(highly recommend for babies) to picture books to early readers to Harry Potter (which my 8 year old granddaughter is currently reading and means we can have her continue reading when she comes for a weekly sleepover).
Plasmacar - probably the most played with purchase we’ve made for grandchildren. Tons of fun!
Several of the Little People playsets - the Learning Zoo, the Animal Farm, and the Garage seem to be the most popular with the kids.
A few stuffies and a couple of dolls - Baby Stella is a good first doll.
Viewmaster
Soccer balls
Colored paper and kids’ scissors
Drawing paper and colored pencils
Coloring books and crayons
Playmat for babies
Play/activity table
Kids’ kitchen, with pots & pans, dishes, cups, food
Toy vacuum
Push toy that can also be sat on (for toddlers)
I also have extras of appropriate sized clothing, diapers, wipes, favorite blankies (they love the Aden & Anais small satin-bordered ones)
We also have a pack and play, a high chair, an infant chair, and a stroller.
I should mention that we have grandchildren who live close by so they are here often. I realize that not everyone is so fortunate. For only occasional visits, one’s ‘collection’ will likely be smaller.
Just my opinion…as a grand kid gets a tad older (so not necessarily toddler) have something that not only is fun but has to be taken care of. My grandmother’s doll (and her mother’s) was a treasure–but you had to take care of it. It was special on a lot of levels but it was expected that you could take care of it. So not just a toy to buy but something special that you trust a kid to take care of.
What I liked best about visiting grandparents or friends was when they had something we didn’t have. I had one friend who had a case full of his old matchbox cars each in its own section. My son loved it and knew it was very special. The other thing I highly recommend if you have room is a really big sand box (8 ft x 8 ft is a good size). Even better if it has access to a hose. When my kids were small they spent hours and hours. My mother had bubbles and those giant bubble wands as another outdoor toy.
We’ll…if you are talking outdoor…get a tub of sidewalk chalk.
My granddaughters love to play dress up, so I have a collection of princess/superhero costumes and accessories. We all have special hats and scarves/boas and jewelry for tea parties. I picked up a cute little china tea set at HomeGoods and they feel very special when we use it.
If we buy them a gift, they get to take it home. If I want to get something new for my house, it just appears in the toy closet.
OP - sharp things and toddlers don’t go together! They make toy versions of most tools. I remember buying mine a toy tool belt they could wear around filled with plastic tools which they loved. Little ones need to learn that adult tools are not a toy they should be playing with. Buy more toy tools, put them in a cardboard box, and watch them entertain themselves … with the box!
@chercheur I should have been more clear, he’s not playing with steak knives of anything, he’s just good with things like trowels and shovels (real ones)
Anything for pretend play was a big hit with both my son and daughter when they were 2-4 years of age. Play kitchen stuff, tea set, a bin full of dress up clothes. We made some menus to play restaurant. The kids also liked a toy phone. They’d use it to call room service or order takeout. Haha. Dolls. We also had yardage of different kinds of fabric that in nice colors and textures that they used their imagination with for all kinds of stuff - making tents, capes, tablecloths, etc.
A child sized table and chairs for crafts, pretend play, games, etc.
Tricycles and other age appropriate scooter type things.
An easel and paint, crayons, etc.
Look online for any Waldorf type toys.
My grandmother was a baker and my mother was NOT. We had little rolling pins and an apron at Nanny’s house (where we were almost every weekend) to help her bake. There were also a lot of special toys that were really too old for us because my uncle was only about 15 years older than my sister so was away at college. There were Lincoln logs and an erector set. Oh, those were so special to play with.
A puppy. Or maybe 3.
When the grandkids are away, the grandparents will still enjoy them.
Another suggestion is an antique style, hand-crank ice-cream maker. Or a sno-cone maker. Only to be used within a half-hour of scheduled departure time. Get 'em sugared up before they go home.
Books, books, and more books.
Since grandparents are often losing their sense of hearing, it’s the perfect place to store the toy drum/bugle.
Right now we have been buying clothing (and returning what doesn’t work based on DD choices but she chooses almost all of what I buy based on what she indicates is needed), and toys that parents don’t seem to notice are needed (they are on a pretty strict budget and a bit overwhelmed) - the 0 - 6 month items for now (GD is 4.5 months old).
When I see stuff, I buy ahead - and getting the kind of things DDs enjoyed as kids. To me it is easier to buy now and store than to look for stuff when the time is right. Many times things I have bought are just not available at other times.
Will see how things go as time goes on. Other grandparents are on a stricter budget too and have another grandchild.
GD is our one and only, and probably will be for quite a while.
Definitely buying ahead is smart, I bought a Bob stroller on CL for the first kid, then when DD was barely pregnant, I spent months looking for a good Bob double stroller on CL, got it several months before the kid was born, but then we used it much sooner than I would have expected so was glad I found it when I did. We do a lot of off the pavement trail walking so a Bob is required, one day I had traded cars with DD and ended up stopping to walk at our place and used her car stroller, that was horrible, so Bob it is!
My DD is so funny, she wants nothing with batteries, no ugly bright plastic, she prefers wood and old fashioned stuff, I’d call it hipster, but she’s not, she just likes that stuff, though she says I am welcome to have any ugly plastic I want at my place 
“My DD is so funny, she wants nothing with batteries, no ugly bright plastic, she prefers wood and old fashioned stuff, I’d call it hipster, but she’s not, she just likes that stuff”
I guess I was hipster. I was the same way with my kids.
The current swing device that works very nicely for GD is battery operated. The one we had was crank up. However ours broke after use with our second child. DD/SIL picked theirs up at a baby/child consignment store - and said it was their best $15 investment…They borrowed one of those fairly expensive rocking devices, but GD didn’t like it.
My sister has 3 grandchildren from one son/DIL who lives about an hour away. She has a playroom set up. They are expecting a grandchild from her other son/DIL who is also not living too far away. I provided (allowed her to borrow) a fair amount of our children wood table chairs/rocking horse/scooter. When the time comes for GD to use, we can collect when we are in her area.
My kids are now young adults and the things that stick in their minds from visiting my mom was first and foremost the unconditional love and total attention they got at her house. When they were alone with her I don’t think they heard the word no (unless danger was involved). They remember the things they did there that we didn’t do with them at home. My daughter’s favorite thing at my mom’s was delving into a pretty box that filled with old costume jewelry that she had accumulated over the years – big necklaces, rings etc. My mom and my son wrote a book together (it never went further than her living room but they had a ton of fun writing and illustrating it) And both kids loved “helping” grandpa with his tomato plants in the garden.
OMG my kids used that jewelry box full of costume jewelry for YEARS - even my son!!!
But did they have the very cool Lobster Earrings, with the screw on backs, like my grandmother had? To go with the fox stoles that had the ‘biting’ mouths. Much better than any battery light up sword.