You live in London and you want to get out of there ? But it's the best place in the world , well after NY .
Buy a bus ticket then travel the city, the ticket shouldn't cost much than $2.5, or you may have to walk back.
$5 doesn't get you much these days. I'd probably buy credit for my PAYG mobile. I dont use it much but I think I only have 20p credit now so $5 would be handy lol.