Favorite thing: The Round Church is right in the middle of town (or close enough) and is worth checking out quickly. It's a very old church (aparently the oldest round church in England?) and it's really nice inside. Although it's not huge, you can spend some time looking around inside...
Fondest memory: The 'round' church or Church of the Holy Sepulchre tpo give it it's scary religious name. I've seen a plethora of churches on my travels of various shapes and sizes but never a round one. Not by any means my fondest memory of Cambridge but worth a photo!