London - some of the happiest moments in my life have been spent there. Apart from what has been said, the museums in London are free (though they ask for donations). And there are plenty of great ones. The British Museum alone takes a long time to take in. Then there's the National Gallery, the National Portrait Gallery, the Tate, my personal favorite, the Tate Modern, and more. It's a great way to spend some leisurely time, take in culture without spending a dime.
Another cool touristy/cultural thing to do that doesn't cost much are the "London walks", a series of themed guided tours:
http://www.walks.com/ I highly recommend the Jack the Ripper tour with Donald Rumbelow.
The indian food is amazing. And if you're old enough, make sure you have a nice lager in a British pub.