I have done a P&O mini-cruise http://www.poferries.com/tourist/content/pages/template/minicruise_minicruises_from_Hull_minicruises_from_Hull.htm
They are £64 BOGOF meaning two people can have 3 days 2 nights mini-break for £32 each...
You can go to Brugges, Zebrugge (sp?) or Amsterdam, basically ferry overnight to get there in the morning, full day in the place and then back on the ferry at night to get home the following morning...