music - it takes your mind off the fact that you're pedaling heavy bikes up many, many hills.
friends - biking is way more fun when you travel with a pack of people. also, you won't look so silly be-bopping along to the music by yourself.
bicycle bell - when there is a pause in the music, you ring your bell (or squeak in my case). and then everyone else rings their bell. if you have a fun bell, you can gain friends (necessity #2). until you soon have a musical medley before the next real song begins. then you stop bell ringing. this is important information to know.
camera - for capturing videos to prove to people (and yourself) that riding bikes on a Friday night is fun and not lame!
comfortable shoes - not hip shoes, comfortable shoes. well, unless your shoes are comfortable and hip, then I'd say you've got this thing in the bag.
a book, any book - just in case. right?