Here's a short list of what some readers have suggested thus far:
"Go to Discovery Park if you want to get out of the city without actually leaving the city. You can spend a whole day walking the trails and mucking around on the beach."
"Walk around Downtown or Capitol Hill with a prop (the more ridiculous, the better) and ask people to pose with it while you take their picture."
"Why don't you make a scavenger hunt list? Put stuff on there that you have to take pictures of in order to prove you did it. Send it to a few friends or have them go with you. You'd be surprised at how fun it is."