I was gonna say scavengar ride (scavengar hunt), but sounds like you want a more organized ride together type deal. The scavenger things could work to get a large number buzzing around a town. Sounds like fun to me.

Halloween.. easy.

Cops and Robbers? If you have a bank with a separate in and out door... have the robbers walk through, followed by the cops. Or maybe just hang outside to get a couple looks and not cause a REAL ruckus.

Cowboys and Indians?

Superhero ride? Dress as your favorite or made-up super hero

Greasers and Pink Ladies? Watch Grease afterwards.

Dress as "what you want to be when you grow up"... can get interesting even for the "grown ups".

Rock the Ride - everyone brings their bike radios (or make some), bonus points for something large enough to require a trailer . Either require everyone be on the same station... or BETTER... have someone rig a transmitter, and broadcast your biking related songs. I know a couple people who've built transmitters, and two of them have the transmitters on their bikes. You can even have someone prerecord the "DJing". Shout-outs, humor, dedications etc instead of JUST music.