you can leave your luggage at the airport and go to london for the day if you fancy it. the gatwick express takes 30 mins to london victoria so you could go and do some sightseeing. it's about 12 quid for a day return ticket.
alternatively, gatwick airport is in/adjacent to a town called Crawley. it's not particularly exciting but there are shops, a cinema etc. it's a short bus or taxi ride from the airport.
finally, there is nice countryside all around the airport and there are some really lovely pubs about. if you fancy spending a few hours in a country pub with a nice meal and some sports on the tv then that could be a good option. i have a friend who lives in crawley so if you're interested i can ask her for the names of some of the better pubs.
