A few places I took my mentor group this year and last year (either just me and two teenage girls or a group of 10-ish mixed gender adults and teenagers):

Frye Art Museum: free, open late on Thursdays. Their special exhibits are usually pretty accessible contemporary art.

Asian Art Museum: free first Thursday nights when the main SAM was closed, now I think their free day is different. When we went there was a big Dia De Los Muertos event and it was a bit crazy.

Canoeing at the UW:
http://depts.washington.edu/ima/IMA_wac.php
$8.50/hr per canoe (fits 3 people), a lot cheaper than renting kayaks, and more user friendly.