My wife and I were staying in Canterbury and decided to take a side trip to Cambridge, not sure what to expect. It was well worth it. It's only an hour train ride from London, so great for a one-day excursion. Take the official City guided tour from the Tourist Information office; you get to enter some of the colleges. The "chapel" at King's college is a must see; it's bigger than many churches or cathedrals in Europe. Attend the boys' choir if your can.
As for punting on the river, I would avoid the guided tours; the one we went on was short and uninteresting. We couldn't understand the guide. If you are able, try renting a boat and go yourself. You can spend as much time as you like.
We had dinner at a really nice pub named the Mitre. Pub food, but very good and big portions.