I regularly drive into Cambridge - poor public transport from here, sadly - and wouldn't recommend it unless you really know where you're going. I second NWIFlyer's PlusBus suggestion - or a taxi is fine too, there's plenty around. Or, my preference if I am arriving by train is to cycle in to the city centre (there are dockless bikes here) but that's certainly not for everyone
Do get the train from King's Cross, rather that Liverpool Street, and in preference to St Pancras. These are by far the fastest trains, and, outside of peak ours, will be first stop Cambridge (they may continue to Ely or Lynn).
Loads to do here, some incredible college buildings, punting on the river, famous historic pubs (read up on the Eagle, for example) and great food. How long will you be around and what do you fancy?