I was recently reminded of the day-return tickets that are available from London to Calais including the P&O Ferry crossings. These used to be quite widely used in the days before Eurostar and the abolition of duty free. The cost is about £26 and the ticket can be used on the Highspeed SouthEastern service from St Pancras as well as the traditional ones from Victoria, Charing Cross and London Bridge. Leaving from St Pancras on the 7.10 train, I was in Calais with time for a 2-hour lunch and a bit of shopping and looking around. There can be a fair bit of hanging arround at the ferry terminal. Calais is not the most picturesque of French towns but it is not uninteresting and has plenty of restaurants. The great advantage of this option is that you do not need to plan it in advance - although getting the ticket the day before is a good idea to avoid possible hitches at the ticket office but after you have some idea of what the weather will be like (it can get nasty in the Straites of Dover).