As said, it is exam term at the moment so most of the colleges are closed to visitors, with the exception of the big / Royal colleges that charge. Most are about £5 to view the courtyards/gardens. We have generally found St Johns, Kings, Queens and Trinity charge and Corpus Christi, Magdalene, Clare and St Catherines are usually closed to visitors for some reason. If you come after 14 June (end of term) it might be better. Queens has a very pretty old courtyard, although the oldest original courtyard is in Corpus Christi (go down from the far left corner of the first courtyard space) – if it’s open to visitors. You can usually wander around Peterhouse (oldest college) and Pembroke (ornate library and Christopher Wren chapel) which are next to each other and 5 mins from King’s Parade. If it’s sunny it is worth seeing King’s Chapel for the stained glass. As said, you can usually get in to the chapels for free for choral evensong or organ recitals during the evening.
Personally, I would look at the three Royal colleges (St Johns, Trinity and Kings) from the outside – you get a good idea of the stonework and grandeur – and wander in to a few the others if they are open.
It is also worth visiting a few of the museums since they are free, depending on what interests you.
Also worth doing a punt for a general overview and view of the backs that you don’t get from the public footpath (Garrett Hostel Lane). EDIT – although I would haggle… a lot.