Cambridge has some amazing colleges and sights that are well worth seeing, but my favorite parts are not as well known. Along Huntingdon Road, going up towards Girton, you pass Trinity Which field. Walk down the passage beside it and by Fitzwilliam College. If you walk down Storeys Way and cross over Madingly Road you can reach the quiet residential areas of Wilberforce Road, the coton footpath (a good walk but longer).
This way can eventually lead you down to Kings College and 'the backs.'
A way along Madingly Road you can visit the observatories. Though you can't go in, they are in a nice setting.
I would recommend visiting these areas if you don't mind wandering around and would like to see the quieter parts of Cambridge.