Having lived in Cambridge (and worked there) for some years previously, none of the colleges save Homerton are particularly near Addenbrookes. All other than Homerton and Girton are relatively near the city centre - even the "west" colleges aren't that far away (I've walked from the West Sites to the city centre before, it's not that far).
Green spaces are pretty widespread in Cambridge outside of colleges and plenty have a fair bit of green space inside colleges. Jesus has a pretty big green at the back I believe and is adjacent both Jesus Green (not actually private grounds of the college but a public park, although I believe owned by the college...?) and Midsummer Common (with cows some of the year!), and is pretty central. No idea what the architecture is like inside though! Emmanuel and Downing also very central and I believe have green spaces inside/adjacent the colleges.
Most of the central colleges have a fair bit of "classical" style architecture (obviously Kings is the posterchild with the famous curtain wall and big chapel but most have similar architectural offerings as well), along with usually a number of more modern buildings. Sidgwick and West colleges are somewhat more recent so varied architectural styles, one of them had a very "collegiate" looking red brick style for example (I want to say Robinson but I forget).
Have a look on their websites and google images, I expect there are lots of pictures available! If possible try and plan a trip to visit Cambridge perhaps - even outside of open days a lot of colleges arrange tours in the summer as I recall?