I am surprised no one has replied to this. Unfortunately there are not many preschools that are not tied to a daycare these days. A friend sent their child to Great Beginnings Nursery School on Querido Drive (860-659-0923) several years ago and loved it but I am not sure if it is still there. Give them a call and see. There is also Cathy's Nursery School (860-633-5726) on East Carriage Drive. I had heard good things about it but that was years ago now. Here is a link to their website:
https://cathysnurseryschool.com/
I do know people that liked the preschool at Aloha on Hebron Avenue and at Bright Horizons at Village Place. Aloha is locally owned by a family in town and its owner was my kids teacher at another daycare years ago. She was great.
For a children's gym, you can try My Gym on Oakwood Drive. I also think that the Glastonbury Family YMCA has programs but you need to check their website for them.
As for swimming lessons, Glastonbury Parks & Rec offers lessons at the high school during the cold weather months and at the town pools during the summer. They do have an infant to 2 program but you would have to check when it is offered. They will also assist you in finding an instructor if you want private lessons. Healthtrax also offers lessons in town.
A great program for swimming is offered at the School of Swimming in Rocky Hill. It begins in May and you have to register early because space is limited and they fill up very fast (like in less than a day). If I remember correctly registration begins in February but I would call them to find out the exact day it starts so you can call that day. If you don't, you will likely not be able to get in. I hope this helps. Jay