Congrats!!!

I suggest the Imperial War Museum, and you just have to take a stroll down Park Lane and check out the posh car dealerships. If the weather is good, spend an afternoon in Hyde Park - it can be very romantic. And go down to Leicester Square and try to get a last-minute ticket for one of the classic plays/shows/musicals.

But all of that, apart from Park Lane, is in the tourist guides, so I am sure you have already considered it. There is so much to do/see in London - it all depends what you are into.