The best option is to pick an area and wander aimlessly. You will be guaranteed to find lots of history and interesting pubs and most likely pretty parks and gardens as well. Be sure to go down narrow streets and alleys, as they often lead to hidden openings or charming mews. Any buildings that have housed famous people or events will be marked with a blue plaque listing who and when.

My favorite areas to explore are Mayfair, Southwark, Hampstead, the Inns of Court, the City, and Richmond for river walks. Another idea is to create your own theme tour. For example, set out to see all the train stations, or bridges, or parks, or markets.

If you are the type of person that wants commentary on what you're seeing, or don't like exploring on your own, there are many options in London for different types of tours.

Bus Tours
There are three companies that offer hop-on-hop-off bus tours. They generally offer several different routes, and added extras such as free river cruises. Tickets are generally valid for 24 hours. They can be boarded at most major tourist areas. I think the best one is the Big Bus Company, because it has live commentary.

Bike Tours
It is possible to tour London by bicycle - either on your own or as a part of a guided tour. London Bicycle Tour Company offers several different routes.

Walking Tours
Several companies offer walking tours and their brochures can generally be found anywhere there are tourist pamphlets. They do get rather large crowds in the summer months, but in the winter they are a great way to learn interesting facts about an area in an intimate group. They generally follow a theme so there is bound to be something to suit your taste. Stepping Out is one of the companies, and has a web site.

Boat Tours
I'm not particularly fond of the river cruises, but many people enjoy them. There are several companies that offer cruises and they leave from different piers and offer different services. A listing can be found at LondonTown.com.