Be my travel agent

I'm headed to D.C. this coming weekend with some friends, and although our itinerary is already full of Important Monuments and Museums, we're also hoping to see some more unexpected stuff -- the kind of bizarre semi-secret destinations that might turn up in an Armchair Field Trip, like the world's largest thermometer or Bubblegum Alley. Any ideas? (Keep in mind you'll probably have to read about them next week...)