1. Head to the library or the nearest bookstore and read for a few hours.
2. Wander around the city aimlessly -- last night was somewhat bearable without a coat.
3. Window shop/people watch at the mall. Union Square would be good this time of year.
4. Find some art openings. First Thursdays would be the ideal time to get locked out -- you'll be set!
5. Pretend you're a student and crash a lecture.
6. Stand in the aisles of Blockbuster or Hollywood Video and watch whatever movie's playing all night, which would really creep them out.
7. Sneak onto Muni and ride around for a few hours.
8. Hang out at a bar and act slutty in order to get drinks bought for you.
9. Find some pennies and make a peace sign on the sidewalk.
10. Find a cardboard box and take a nap, or do some breakdancing if you can't sleep.