I'm a student there at the moment, and well, the university itself is massive - you definitely won't be able to cover it all anyway. There are 31 colleges, so you'd probably want to pick a selection. The colleges' locations differ greatly too, for example, Girton is located a good 4km outside the city centre. Popular tourist spots include the older colleges such as King's, Trinity, and St. John's. Perhaps you might want to visit a few of the newer ones too (such as Hughes Hall, Churchill, Fitzwilliam) just to get some perspective. It varies from college to college, but some of them don't take very well to tourists, especially when they say they're closed (which in some cases, is just about always). Some also charge entrance fees - King's charges for chapel entrance, St. John's charges for grounds entrance, and I think Queens' does as well. For free and good views, take a walk along the Backs, the other side of the river, and you can get a decent view of some of the colleges. There are also plenty of green spaces - my personal favourites are Jesus Green and Midsummer Common. The latter actually extends extremely far down the river, and you can follow it for several miles. It's out of term now, but if you're lucky, in the mornings there are sometimes rowers in the river, as the boathouses are all located there. There are a few good museums - the Fitzwilliam Museum would be an obvious candidate, and then there is a rather nice (and free) Zoology museum at the New Museums Site. It's right underneath a massive whale bone thing, you can't really miss it. Most tourists end up going punting on the river - there are tours, and you can also punt yourself, albeit with the added element of possibly falling in the water. The tours however are not entirely accurate, and I've heard some rather suspect commentary.
Just outside Cambridge there is the village of Grantchester, with a popular tea garden called The Orchard. Grantchester can be reached by bike (30 min), walking (1 hour) or punting (depends how skilled you are, anywhere from 2 hours to many hours). The timings are for one-way trips. I have heard that there is a bus, but I'm not very sure. Grantchester Meadows is rather picturesque - there is a Pink Floyd song bearing that name, and it's also the subject of a Rupert Brooke poem. However, this is entirely outdoors, so pick a good day to go because if you get caught in the rain you will most certainly get soaked.
And if you're interested, you can check out my thread of Cambridge photos here:
http://www.clubsnap.com/forums/land-city-scapes-travel/1082275-cambridge.html
