I think that you might have to restrict yourself to this country. You can get a day's unlimited rail travel (after 0845 Mon-Fri or anytime at weekends or Bank Holidays), throughout East Anglia, for just �11. Even better, you can travel on any 3 days out of 7 for just �23. (If you've got children travelling with you, they go for just �2 each, whether it's for one day or for three):
http://www.onerailway.com/templates/Generic.as px?id=2197
(If you need suggestions about where to go in East Anglia, just ask. Real ale pubs a speciality!).
Alternatively, why not explore London? At the moment, 'One' are offering a single fare to Liverpool Street, on 9th May, for just �11 on all morning trains from 0800 onwards. (The 0800 service arrives at 0956). If you return at 2130 (arriving back at 2327), the single fare is just �6, so that's a total of �17 return. (If you wanted to return slightly earlier, it would cost you an extra fiver). An off-peak Zone 1&2 One Day Travelcard (giving you unlimited travel on buses & tubes within central London) costs �5.10.
If you need suggestions for some of the great free attractions in London, just ask. (I can also recommend a restaurant, in Covent Garden, which serves a really good 3 course lunch for just �7.50).
I'm sorry I couldn't find any cheap foreign travel offers but I hope that something here might be useful to you. I'll keep an eye on this thread to see if you want any further information.
Chris