You can do open bus tours which are pretty cliché, if your planning on it being a nice day buy a couple of tickets, their like 20quid compared to a 7.50 rail pass, but will drive you through all the "typical" torist'y joints in London, bus's come all the time like every 5 mins and its pretty cool, it can be as long or short as you want to make it, you can jump off at every stop or pick the places out you really want to visit, i.e. Hamleys, Southbank, blah blah blah, you get a free river boat to greenwhich which is pretty cool, lets you venture about without having to get on the underground.
Not saying it will be your cuppa tee but something to look up if your interested, think "the big red bus company" was the cheapest last time I did it.