London is a huge place for every kind of activity, and there is just too much to be listed on a single page here. However, here is a list of experiences and activities you can do in London on any kind of budget:
Top 10 Free Things to Do in London - a list from About.com covering some of the main activities in London you can follow for free.
101 Things to Do in London - a great list of things to do from Time Out, starting with a knees up and ending at a raw food restaurant!
London City Guide - a guide to the more practical questions for those visiting London, such as where to find childcare, to finding the best Indie shops. A good way to move towards what you're really looking for.
Things to do in London - more a refresher covering a lot of the above, but a nice guide in itself to the various monuments and locations across London for visiting.