Brighton is highly recommended as there is so much to do. However, go by train as the traffic as the weekend can be terrible - can take up to two hours on busy Saturdays/Sundays. However, if you did go at the weekend and got stuck in traffic, you could change your plans and go to the Blue bell Railway at Sheffield Park.
Or if you want a really short drive, why not go for a walk in the woods at Beckenham Place Park?