I think it really does depend on the weather as well, and how much you are prepared to spend on a taxi

Brighton is a great idea, but as previously mentioned only really enough time for a quick look and then back again. Not really worth the taxi cost in my mind

Another option would be Horsham. It's only about 10 min by car and has some lovely old streets, independent restaurants and shops to look around.

However, as I said, if the weather is bad it might be better to stay in the airport. You could also head to Crawley and go and see a film at the Cineworld complex if the weather is bad...