You have a wide variety of day trip options out of London:

1. Windsor (castle tour)
2. Dover (another castle, and the white cliffs)
3. Cambridge or Oxford
3. Bath or Salsbury (I've toured the baths there before, and think I'd opt for a castle tour in Dover or Windsor if you only have the one day.)

Just sort of depends what your interests are. All of these are accessible by train from London.

If you don't mind a 2 hour train ride, you can even get up to York. The cathderal there is worth the trip alone, and there's a pretty decent rail museum there also, right next to the train station.