I know Cambridge quite well.
As has been indicated above, the colleges that make up Cambridge University function largely independently, with their own rules They're also quite well spread out across the city, with several miles between some sites. However visitors are normally welcomed to the courtyard areas (etc) of the central historic colleges except when examinations are taking place. (Unlike most other universities, Cambridge does have some examinations taking place in mid-summer but you're probably safe to visit most colleges around now).
However what visitors get to see isn't representative of university life in Cambridge, as the majority of students live and study on modern campuses (such as the West Cambridge site), rather than in the historic buildings.
As Eddie says, you can get day tickets on the buses in Cambridge but, as you'll be travelling by rail, it's better to but 'PlusBus' tickets when you purchase your train tickets. They give you a day's unlimited travel in Cambridge for £3.20 for an adult (or £2.10 for a railcard holder) and £1.60 for a child. As it's a good 20-minute walk into central Cambridge (and several miles between some of the college sites), it makes sense to use the buses. Service 1, 3, 7 and 8 all run from the rail station to the city centre. (On leaving the station, turn left and cross over the road. The stops for those services are at the far end of the row of bus stops).
You ask for a good restaurant in Cambridge. I suggest the Club Polonia, which serves some of the best Polish food you can find in the UK. Main course menu here:
http://www.klubpolonia.co.uk/files/dania%20g%C5%82%C3%B3wne%2009%20June%202015%20%281%29.pdf
It's on the junction of Chesterhall Road and Chesterton Hall Crescent and is quickly reached from the city centre by taking a No 2 bus (with its destination shown as 'Science Park'). Reviews here:
http://www.tripadvisor.co.uk/Restaurant_Review-g186225-d3170015-Reviews-Polonia_Club-Cambridge_Cambridgeshire_England.html
Punting is fun but expensive. If you're feeling lazy though, there are punts where someone else (who is often a uni student) will do all the work for you.