We had a surprise 3 night trip to London on our way home from Greece this summer. BA at LHR went on strike the day we were headed home (through LHR), so we made lemonade and stayed the 3 days it took BA to get us on a flight. It was fun, we saw Lion King and we stayed at the nicest hotel I've ever been in (even better that a Ritz, or Four Season IMHO), The Renaissance Chauncery Court (I think thats what it was called)
We did have to spend 7 hours in the airport before we got all of this figured out, but it was still a fun jaunt.