mmmm...restaurants:
I like:
Cotto - East Road near the grafton centre, small restaurant, lovely food, only eaten there once though, but had a lovely meal there - quite £££, probably need to book on a weekend.
Backstreet Bistro - this is near the Mill Road area, bit cheaper than Cotto, nice relaxed atmosphere
Midsummer house - Michelin Starred, lots of £££, possibly too late to book, been once, food was amazing, but not sure I would go back soon as it was so expensive
Restaurant 22, lovely, intimate, tiny restaurant. You'd need to book here too.
Places to go, if it's nice weather, the botanical gardens are worth a wander.
Punting of course. You could take a bottle of wine and go down towards Granchester and do it yourself, that would be romantic and nice.
Or walk to Granchester and go to the Tea rooms there which are lovely.
Kings College is spectacular and worth paying to look in.
Pubs, I live near mill road so like the pubs down there, would recommend the Cambridge Blue if you like real ale, lots of ale on tap.
The Kingston Arms if you fancy pub food and good real ale (they don't serve lager though! Though they do have good wine I believe, I've never tried)
In town, the pubs up towards Castle hill are quite good, the Pickerel and the Castle being two I would recommend. Also the pubs down King St, the St Radegund is tiny and quaint and worth a visit.