1. The colleges are all fairly good although they all have their stereotypes. Choose whether you want catered or self-catered. One self-catered college (Fenner) is off-campus, which isn't all that convenient (it's pretty far to walk, although I think they have a bus that goes to and from the hall). I live at Burgmann (catered) the food isn't great, but it is good. The rooms at Ursula and the self-catered colleges are fairly tiny. Apparently Johns and Bruce have pretty big rooms (I've never been) but I wouldn't recommend going to Johns unless you lack a neck and really like rugby. But in all seriousness, you'll meet great people regardless of which college you go to.
2. There is stuff to do. There are so many students around that there always seems to be something on. And there are some good places to go to. There are a bunch of clubs/bars if you're into that (Mooseheads is a popular haunt, but I prefer the Kremlin Bar), there are shops nearby and a cinema, lots of restaurants, all these are in walking distance. Even if you just stay in college, you can watch a movie or whatever on other peoples computers and stuff. You'll probably also have work to do. You won't be bored, in any case.