As has been said, no problems getting between London and Cambridge with trains every 30 minutes or so at peak times and you can't make reservations on them (but you must buy your ticket before getting on otherwise you'll end up with a £10 each). Getting to Waterloo for the Eurostar is no biggie either - short tube journey or a cab.

Cambridge to London should be a little over an hour to 1:30. Transfer to Waterloo for the Eurostar depends on whether you arrive at Kings Cross or Liverpool Street. Have a look on the journey planner on http://www.tfl.gov.uk and you can plan your journey pretty well (although the journey planner seems to be down at the moment). I'm guessing you'll need about 30 minutes to get to Waterloo.

You need to check in for Eurostar trains at least an hour before departure. You will have an airline-type ticket. The security checks are tougher now too so take the time to get through security into consideration (it's nowhere near as long a wait as airport security)

As for sites in Cambridge, the universities are interesting to walk around, and a punt on the river is good if the weather is nice