In my student days I scaled Green's Windmill vanes and the sandstone cliff by the castle to get in to Mortimer's hole for a laugh. Used to have a wander round the Raleigh site and Players too, but they're all flats and houses now. How about an explore of the railway tunnel under Victoria Centre.
Take her to an anti war protest in London, ideally within 300 yards of the Blair residence.
Heavy pet in a public swimming pool.
"Surprise" sex?
go to a restaurant and do a runner.
go to a big multiplex cinema when its not too busy. Spend 1/2 hour in the film then move to another. See how many screens you can do.
go shopping in town with the mission 'who can steal the funniest thing'. It only counts if the other person doesn't see. If caught just plead accidental a la Richard Madley.
add mind altering drugs for the extra edge. Who can spike who with what without them knowing is good too.