Visit london eye. Best to book via the website
http://www.londoneye.com/
Recommend walking haunted london tour. Get to see some obscure parts of london
http://www.london-ghost-walk.co.uk/g..._Jones_MMC.htm
Goto the west end to see a play.
Depending on your time you are there, look out for any music festivals. I think there is one in Hyde park sometime this summer. (Or even the Olympics in 2012!??)
And yes, bring your wallet, it can be expensive, but how often will you come to London?