Cambridge, UK--Great things to do?

Cambridge is made up of 31 different colleges scattered around the city - have a look at this illustrated map - so you won't have time to visit them all! Pick out two or three to concentrate on. AI picks out the following top colleges for visitors. You will need to allow at least an hour for each.

King's College:
Famous for its stunning Gothic chapel, a masterpiece of late-Gothic architecture with large stained-glass windows and a fan-vaulted ceiling. It is also centrally located.

St John's College:
Features impressive buildings, beautiful gardens, and the famous Bridge of Sighs. It is also the only college with two bridges.

Corpus Christi College:
Known for its historic Old Court, peaceful courtyards, and the world-famous Parker Library. It was founded in 1352 by local townspeople.

Clare College:
The second-oldest college, with some of the best gardens in Cambridge situated next to the River Cam.

You will need to check on opening times.

The Botanic Gardens will take 1-3 hours depending on how quick.slow you are...

Lots of companies offer punting - allow about an hour for this.

Evensong at King's College Chapel

In addition to visiting the colleges, you might want to go to The Eagle and Child (now just The Eagle) where Watson and Crick worked over lunch to help understand the protein-coding nature of DNA. Look at the history of airmen who stayed there during WWII.
Fans of the TV series, Grantchester, my husband and I walked along the backs of the Cambridge Colleges to the village of Grantchester and stopped along the way to have lunch at the Orchard Tea Garden. Cambridge is a lovely town which much history.

I lived not too far from Cambridge for five years and visited it regularly. I loved to wander the streets, exploring the colleges and the market area. The college area of Cambridge is very compact, flat and easily walkable. Basically follow Trinity Street from the Round Church down to the Fitzwilliam, veering off into the narrow streets to explore the colleges, the market area or the River Cam and the Backs. I was impressed with the Fitzwilliam Museum and recommend it but never visited the modern art museum. I also never punted but did enjoy standing on the bridge watching the punts go by. Kings College Chapel is pretty, especially the ceiling. Since you will be staying with friends I am going to assume you or they will have transportation and suggest some relatively nearby places you could consider visiting at least for one of your three days. Longstanton is not far from Ely with its amazing cathedral, standing watch over the fens. Ely also has a cute riverfront area with lots of narrow boats. Right outside of Cambridge there is a National Trust property, Anglesey Abbey, that is scenic and peaceful with its 98 acres of gardens, manor house and mill on the river. Wimpole Hall, a huge stately estate, is also near Cambridge. If you like World War II history or aviation visit the Imperial War Museum at Duxford and you will be in aviation heaven. There is also an American military cemetery between Longstanton and Cambridge. A little farther out, about an hour's drive from Longstanton is Lavenham, a gem of a small picturesque market town full of wonky old timbered buildings. Lots of tourists visit Cambridge but the surrounding area has many places of interest off the beaten path.

Thanks every so much, all you who have replied to my questions! You are so generous.