Help for Camp Grandparent

My 10 & 14 year old will be visiting their grandparents mid-summer. I'm looking for some outside-the-box things to do, as they do this every summer and there are only so many times a kid wants to go to the aquarium. Only caveat: My mom can't walk long distances, and managing the beach is out.

(notes: yes, they already spend time just hanging out, which the kids love; my mom in particular would like to just not go stir crazy in the house. Yes, I've got the Riverdogs, looking up classes at Michael's or other craft places, and the Astronomy observation at Brittlebank park. Library has not listed things for the summer yet. Unfortunately, Charleston Cooks is closed, or I'd suggest a cooking class - if you know of other ones that a tween or teen could do, that would be great).

Thanks for ideas!