We lived in Bethesda for about a decade until very recently. I scoured this forum "classes, workshops, camps...etc" that whole time and found the following to be some of the best places for camps / out of school experiences for ES and then early MS kids and I thought I would share this information in case it is helpful for others.

Landon Summer - the specialist camps like fishing, ceramics, painting, food resources (SSL). My kids were not particularly sporty and didn't do the ES level camps here.

Valley Mill - we found this much more comfortable an experience than Calleva which was a bit rough and tumble (carry your own kayak when you're only 10 etc). But the Calleva horse riding camp was definitely worth it. Otherwise VM was excellent, and my dirty, tired kids would come home every night having swam, made stuff, totally busy and worthwhile.

Kid Museum - for us this was fantastic for SSL hours when my kids were in 6th grade. They went to a wide variety of charitable places, soup kitchens, rehoming charities, took lunches to bed bound people living with HIV. It was remarkable.

Sidwell Summer - this was slightly more mixed an experience, depending on the quality of the junior campers but the sewing camps, the building go-carts for disabled kids etc were all really worthwhile experiences.