Are there any C.S. Lewis sites at Cambridge?

Not sure what you mean, but Cambridge was Lewis's home for much of the latter part of his life. He actually was the first professor of Medieval and Renaissance literature at Cambridge. If by sites, you mean places of interest that he frequented, there is a C.S. Lewis Walking Tour available in pamphlet form once one reaches Oxbridge (home of Cambridge). I found the pamphlet information at the following link:

cslewis.org/programs/oxbridge/2011/CambridgeGuide.pdf