If you can find some of the special offer fares on Eurostar, then Ostend might suit your needs. (Fares from London to Brussels include onward travel, within Belgium, free of charge).
You used to be able to get there on Hoverspeed, originally directly from Dover and later via a coach connection from Calais. Unfortunately, Hoverspeed, have effectively ceased trading as they stopped taking new bookings from Monday of this week. The only other way to get to Ostend cheaply would be to seek out cheap flights to Brussels and then take the train. If you get bored with Ostend (which is full of bars and nightlife) you could always jump on a tram to its more downmarket neighbour, Blankenberge.
Chris