Oxford would seem to be a good option - at peak times you've got 10 express coaches an hour from Victoria! (6 Oxford Tubes, 4 Oxford Expresses). It'd be slightly quicker to take the underground (tube) to Paddington and the train from there though. If you get off a Queens Lane / High Street, you'll be surrounded by colleges, not too far to walk to some of the parks, and close to shops

Otherwise, take the train to Brighton, shop and wander round in the Lanes, walk on the sea front etc