Visited Mystic for 3 days last week. No trip report necessary, but just some potentially helpful bullet points.
Stayed at Mystic Marriott. Looks kind of rundown from the outside but the inside (including rooms) is recently renovated. It was downgraded a category with the merger. (I had a back and forth before the stay about getting points back, they canceled my certificate, but never reissued. Points are still in my account after the stay 🤷♂️)
There are some nice beaches around, some very secluded. Beware of parking fees. Esker Point Beach in Groton was really nice, free parking and quite empty. We went late afternoon on a cool day, probably more crowded mid-day when it's hot.
We spent a day and a half at the seaport (on a single day admission--save your receipt!). Plenty to do there for kids.
We spent our last evening at Bluff Point State Park. Gorgeous picnic area on the water with views of the sunset, and an airport runway right across the water. We were lucky enough to see a fleet of military jets talking off (no idea what they were doing there!)