Cambridge:I'm a little impartial here as this was my home for a summer, but i think Cambridge should be on everyones list of side trips from London. I think the city has much more atmosphere than did oxford (although i was only in oxford for a day). From London you can take a bus or train to reach Cambridge. National Express offers buses and National Rail for trains
To understand Cambridge you must understand how the university system works. Within the University of Cambridge, there are several colleges. These colleges are essentially self running and students live, eat and socialize with others from the same college. There are also some serious rivalries between the various colleges. Depending on the time of year it may or may not be possible to tour the colleges. During exam time they are closed off to visitors. During the rest of the year you probably will be able to get in. As an unofficial word of advice, if you act like you belong there when walking through the gate you should have no trouble getting in.
If the weather is nice I recommend renting a "punt", a flat bottomed boat steered with a pole. and doing some punting along the river Cam. Later in the evening you can have a pint at The Eagle pub where Watson and Crick first announced they had discovered the structure of DNA. Across the street from the eagle is St Benet's church, which dates from the anglo-saxon period (1000AD). Another pub of historical significance is the Anchor Pub, located along the river. This is where Syd Barrett, founding member of Pink Floyd, played before he hit it big.