Why would you worry about Mexico in summer?
Not a resort, but one of the best mini-vacations I took was with girlfriends. We flew in from different parts of the country to a major city. Stayed at an AirBnB for a long weekend. We didn’t cook, but we stocked the kitchen with yummy low effort snacky foods and wine from the corner grocer. We ate some meals out at inexpensive places. It was a weekend of board games, talking, and much laughter.
Have you looked at RoadScholar.org for any trips that might interest the group? There are a wide variety of trips, and this would be a different version of “all inclusive”
I think you have too many good options! Good luck!
The Sagamore in Bolton Landing is absolutely outstanding. I have been there several times with DH for conferences and we went for a Mother’s Day Weekend about 2 years ago. The grounds are lovely, nice indoor and outdoor pool areas, spa, the lake itself is beautiful and there is a boat trip with narration that teaches you many facts about Lake George. Depending on the time of year there is golf, beautiful hilly course. If in the fall, foliage season would be breathtaking.
Also on Mackinac Island is Mission Point Resort. Another island consideration is Beaver Island and the lodge or private home. For old fashioned summer is Weathervale Resort south of Frankfort and the Portage point inn in Onekama or something modern like The Homestead Resort
I think many of the places mentioned here are not AI, which the poster was looking for. Anytime I go on a girls’ trip with more than 3 or 4 of us, AI is preferable. Everyone can sleep in or not, eat lunch by the pool when they get hungry, grab a cocktail at their whim, all without compromising with anyone else. Our general rule was reservations at 8 pm for dinner for the entire group, other than that it was very loose. This can only be accomplished at an AI where you don’t need transportation and there’s lots to do on site.
Another possibility in Vermont (I am counting as “all-inclusive” any place where you can book accommodations including three meals daily):
NEPatsGirl - where have you gone on your girls’ trips ?
I’m wondering - with a group of 6 women will we feel out of place at an adult only resort? We prefer to avoid a lot of kids and families but maybe it will be uncomfortable to be at an AI resorts with mostly couples. AI’s are new to us so at this point we’re just asking questions and trying to figure things out. Thanks 
@NEPatsGirl I agree. OP, I sent you a pm with my recommendation and experience with an AI
I’d avoid Mexico in the summer because of the heat. I’ve been in the summer before and it was hard being outside doing things during the day.
Thank you NEPatsGirl - I’ve been looking at that information. It looks good but there are a few questionable reviews. I know reviews are subjective …so it’s certainly worth considering.
@rightfit, the AIs I’ve stayed at include Royal Playa Del Carmen 4.5(walk to 5th), GR Solaris 4, Secrets Vine, 5* Royal Cancun 4.5* , and a few others. I’m not a fan of AI unless its a group trip. Which had questionable reviews? I’ve stayed at several others just can’t remember the names. FWIW, for non-AI vacations my favorites are St. John and second is a small island off Cancun that entails a 20 minute ferry ride, Isla Mujeres. I try to get there at least once a year. If you stay in Cancun make sure you do a day trip over. There are three AI on Isla, two I would recommend, Privileges Aluxes and Mia Reef. The Palace is too far removed from centro and too uppity for my taste. Exchange rate is nearly 20:1 which makes non AI a bargain. Bucket of beer on the beach will run you less than $5 US.
I second Secrets the Vine that @NEPatsGirl recommended. We had a great time there. Food, people all excellent. Just let them know upfront you are not interested in their timeshare they try to sell. While they don’t call it timeshare it’s the same thing.
I also recommend Hyatt Ziva/Zilara, Rosehall Iberostars Suites in Jamaica. While the Secrets resort there is a nice facility, I understand the food is not as good as Hyatt and Iberostars. The Sandals properties give a great island feel and you are able to do “Stay at one play at all” hopping from one sandal to the next on the island.
I always return to Rosehall Iberostars suites as the staff has become like “family”.
^Ahh, yes, I forgot about Jamaica AIs. I’ve stayed at a couple there. What I don’t like though is not being able to venture out off the properties. In Cancun and PDC there’s so much exploring you can do without it costing a fortune or involving a tour.
Do not worry about going to an all adult resort with single ladies. There are lots of adult families, groups of singles, and even solo travelers. It seems the adult only’s crank up the food quality just a bit more and you’re not around lots of kids and kid activities.