Depends how much you want to spend and where you are.
I did a hot air balloon ride for my daughter's birthday one year, and we have done Tea at The Ritz which is a lovely experience. There's so much to do in London that you could incorporate so many things, such as London Eye, open top bus tour. river cruise, duck tour etc etc, but thats no good if you cannot get to London easily.
You can do a VIP tour at Longleat or Woburn which are both really good visits, and they take you close up to the animals.
Perhaps an idea of where you are based would help and budget?