Music - Seattle has a great kids music scene. I recommend anything by Recess Monkey (which my kid loves), Caspar Babypants, or the Not Its (all from Seattle). Or, They Might Be Giants has wonderful kids albums, and as I suggested to someone else, Elizabeth Mitchell has the loveliest CD's for children, and your sister will appreciate her as well - just lovely, gentle, toddler friendly music, great for decompressing kids and moms both. Perfect music for a 2 year old. A great gift for around $10, and completely unique and special.
And maybe have someone take your picture doing something goofy and in Seattle, like feeding seagulls on the ferry or eating mini-donuts at Pike Place Market, frame it, and the next time they come to visit you can take him to that special place.