One of the best places I took my kids (now grown) was the Enchanted Woods at Winterthur. There was also a discovery room (may only be available on certain days) which my kids enjoyed. Skip the house tour, but you might check on the current exhibits and see if any would be of interest.
https://www.winterthur.org/fun-for-families/
We did it as an overnight - spent a day at the beach, overnighted near Winterthur, then spent the next day at the gardens and drove home. With a couple of extra days, you could explore the area or do something in Baltimore (MD Science Center, zoo, aquarium). You might even be able to spend some time at Assateague/Chincoteague (I haven’t been yet, but you might want to read Misty of Chincoteague with your child if you go).
The MD Renaissance Fair is also lots of fun and is held on weekends through the fall, starting August 23. Be aware, though, that there are no flush toilets, just port-a-potties.
You might consider Pennsylvania. I haven’t spent much time there, but there were lots of places I wanted to take the kids. I’ve heard the Franklin Institute in Philadelphia is terrific and there are all the historic landmarks, there’s Hershey Park (which was great when I went several years back, although I didn’t do the water park or the zoo) along with Hershey’s chocolate factory in Hershey, and Easton has the Crayola experience.