Pushing a slender piece of hollowed wood up the river with a pole is one of the most popular ways to spend a sunny afternoon in Cambridge. There are numerous companies that will rent you a punt on the Cam for hours, days or nights.
If you hire from Bridge Street you can punt up and down the backs from the Jesus Green lock to the Mill Pond by the Granta pub.
If you hire from the Mill Lane (by the Silver Street bridge) you have the choice to either hire on the lower part of the river (the Backs) or the upper part of the river where you can punt to Grantchester, an idyllic little village just outside Cambridge with a couple of pubs and an orchard tearoom frequented by famous writers of the past. This route takes you through some wonderful river scenery (straight out of something like Wind in the Willows) to the Grantchester meadows where people picnicking among the cows grazing is a common sight in the height of summer. Beware though: cycling to Grantchester takes about 30 mins, punting takes a lot longer (about 2 hours with a good punter or 4 hours if your skills are a bit rusty!) so make a whole day of it and take a picnic and drinks.