When I drive into Cambridge, I usually use the on-street parking bays on Newnham Road, next to Lammas Land. (It's metered parking during the day but free in the evenings). It's only a short stroll into the city centre.
If you're unfamiliar with finding your way around the Cambridge ring road, the easiest way to get to Newnham Road is to go past Cambridge and then 'come back on yourself'. (I'm assuming that you're coming from somewhere around Norwich, as I seem to recall that you live in Norfolk).
Route:
A11 to A14 (Westbound)
Go past the Cambridge exits and take the turn for the M11 (Southbound).
Leave M11 at first exit (Jn 12).
Follow A603 into Cambridge.
As you arrive on the edge of the city centre, you come to a set of traffic lights, where all traffic has to turn left. The road you turn into is Newnham Road. You need to park on the right-hand-side, so it's easiest to go the roundabout (at the end of Newnham Road) and double back.
To walk into the city centre, walk back to that roundabout and turn right. A short distance along, you'll see a toucan crossing. (That's a crossing designed for both pedestrians and cyclists). Cross the road there and go straight on, following the (well-lit) path across the meadows. That takes you into the city centre. It's under 10 minutes walk to the Corn Exchange.
Chris