Fowlmere Nature Reserve is an RSPB reserve south of Cambridge, with reedbeds, chalk streams, pools, and quiet paths for birdwatching. It is a gentle, low-key place rather than a big-ticket sight, but that is part of the appeal.
Come for a slow walk, especially if you have binoculars and want a break from colleges and museums. The reserve is open all day, with donations requested from non-members.