Replying to myself here about the summer camps. D22 did a one week residential camp this summer at a school that is on her final list and it was fairly expensive (though definitely not as expensive as college!), but another camp we did summer after freshman year was just a daycamp and was FREE! So look around and see what you find.
A lot of colleges participate in the Splash! program one weekend in the spring and fall where college students teach classes to high school students. Some of them are serious and some are silly like Philosophy from Harry Potter or something along those lines. Check out colleges with Splash programs here: When & Where? :: Current Programs A lot of them were impacted by COVID last year but hopefully they will be up and running in person again soon. They are free or cheap. I think the one local to us was $20, $30 if you wanted a t-shirt.