Victoria Line to Kings Cross then train (1 hour) to Cambridge. Buses from outside the train station to the centre, or walk (1.5 miles ish). If walking, it's nicer to exit through the car park and walk into town along Mill Road (check google maps).

Walk round the colleges, try your hand at punting, buy some stuff at the market, visit a couple of the central museums if you like, etc. Have a pint at the Eagle (Bene't Street), look into the back room and see the ceiling graffiti from WW2 US Army Air Corps pilots (contrast with part of the back room which is Elizabethan (or similar age).

On the way back to the station, there's good food near the station off Mill Road at the Kingston Arms or the Cambridge Blue. Al Amin Asian grocers do the best fresh samosas at the counter in the back.