I thought the Eurostar was great, from Waterloo to the centre of Paris. You can go on a ferry any time, but not a 300 kmph train. Paris is a definite must see. But you may find it cheaper to fly to Paris on one of the cheap airlines. I'd also recommend Dublin for a weekend trip by air.

You won't be far from York either which is well worth a visit. If you like a bit of nature, scoot over the Manchester and up into the Lakes District. Very pretty.

London rocks, lived there for 3 years and still didn't see everything.