Places we know & like personally:
Thrive Art School in Ravenna & Madison have great art camps all summer long, but they're short (3 hours+), and kind of pricey (185)... <grinning> at least that's pricey in my book. I may just have camp sticker shock. We do the regular classes with them all year.
Here's the camp schedule:
http://www.drawingschool.com/classes.aspx?ct=1
Seattle Gymnastics Academy in Lake City also does GREAT camps (I know you said he's not a sports guy, but I thought I'd pass it along). The staff is killer, & the gym fun and totally not competitive. (Ahem, they do compete, but that's not their focus...their focus is on having fun and moving your body and being safe). We've done regular classes and camps and parents nights out with them.
Chelsea House Montessori (Ages 2.5-6...so it would be for your middle one) They're my favorite preschool on the planet, and they also have an open enrollment summer session. I don't know what the focus is this year... the 3 years my son went the focuses (foci?) were; Archeology, The Human Body, & Dinosaurs.
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Camps we're looking into, but haven't tried (yet!):
Drama Camps:
Bathhouse Theatre
http://www.seattlepublictheater.org/education-programs.htm
http://gocitykids.parentsconnect.com/attraction/seattle-p...
Stonesoup Theatre
http://www.stonesouptheatre.com/summerprograms.html#top
Science Camps:
Pacific Science Center
http://www.pacsci.org/camps_workshops/
Aquarium
http://www.seattleaquarium.org/NetCommunity/Page.aspx?pid...
Everett's Imagine Children's Museum
http://www.imaginecm.org/page.php?id=53
Great question, btw... I'm going to check back here freq to see what other people know of!!! :)