Fabulous idea! I used to have 5 pound days with my friend's daughter, when I lived in London. Go to Portobello Road, find the Frenchman who does 'banane-choco' crepes, go down to St-Martin-in-the-Fields and see a free concert at lunchtime and maybe have a nose in the market next-door, or wander down Picadilly, have some chestnuts from the vendor in Picadilly Market... Oooh, lots to do in London, cheap and on the fly! Enjoy your time off - I do believe you've earned it!