Off the top of my head:
London Eye and wander along the South Bank
Covent Garden market - good shops, good market and watch the buskers
A walk through one of the London parks - St James's is my favourite
If you like art, the National Gallery is good, but don't try to do it all - there is a good 'highlights' tour
national Portrait gallery (just round the corner in St Martin's Lane) also interesting
Liberty's is a nice posh shop to visit (Regent Street) - I prefer it to Harrods but of course Harrods is a bit of a tourist must-do <shudder>
Museums - national history, science, imperial war, courtauld gallery - it really does depend on how you want to spend your time.
The 'Duck' tour is fun
The Official London Ducktours
What about a guided walk - there are loads on this site
London Tours - Visit London - we did a jack the ripper one which was fun, but there are hundreds to choose from
Actually that visitlondon site has loads of stuff to look through - some interesting ideas there I think.
HTH
Liz