Go and visit HMS Belfast (Big warship floating on the Thames). I spent an abolutely brilliant afternoon in its bowels, oddly it was much more enjoyable than the science museum.
On a sunny day it's also quite nice to just walk along the south bank of the river from the Houses of Parliament to London Bridge.
I'd also recommend the London Transport Museum at Covent Garden.
Although I'm not a big fan of the touristy stuff: Take a guided tour in one of the Routemaster buses - these are actually quite interesting.
Go and get lost in Hyde Park
. Enjoy the Tube - yes really it's brilliant for a tech geek.
Kew Steam museum should be worth a visit, hadn't had time to visit last time I was in London.
Do not go anywhere near the City of London - really awful place
.