There are not too many places in the world where it is acceptable to come dressed as anything at all, or simply naked. Technically speaking, Bay to Breakers is the oldest consecutively run annual footrace in the world.
It is a 12K race (if you choose to race), though it’s something between a parade, Halloween, thus being a spectators or cheering on the sidelines is also perfectly acceptable. The race has been going on in the “City by the Bay” since 1912. Taking runners, walkers and the drunk people following along from the Embarcadero to the Great Highway, the 2014 race numbered at around 45,000 participants and nearly 100,000 spectators the Bay Area and around the world.
The race begins with a tradition (it is unclear to me where the tortilla tradition comes from…) of throwing tortilla’s which can fly somewhat like an ultimate disc (or Frisbee). The run goes through the city from the bay side to the ocean side (hence “bay” to “breakers”) and many people dress in coordinated costumes, while others choose to run naked.
Speaking from personal experience, it is a rather enjoyable and entertaining run for any 12k novice.