Been to the Tower, seen the London eye, bought the t-shirt with the union jack on it? There is a lot more to London than the guide books (even the best of them!) will include. South London is generally ignored by the tourist industry: it has few tourist draws, but a trip of select parts will take you on a journey into what it's really like to live in London, work in London - it will give you an insight into what the city is really about.
Take the underground to Victoria tube station and follow the signs for Victoria mainline railway station. Victoria is the gateway into the south (as you may know if you've come to London via. the Gatwick express). The building itself is one of the many spoiled architectural pleasures of this city. Look up to see the dusty, dirty, but none the less beautiful Victorian roof. Best to ignore the mediocre shopping facilities, although if you explore long enough you'll come across a good cheese shop!
Find a train that stops at Clapham Junction station on the giant billboards: they come every few minutes so you won't be waiting long! Take the train, which should only take you a few minutes. Clapham Junction is, and always has been, one of the busiest train stations on the railway system: now it is depressing and crowded - pass quickly through the shopping centre to the exit. You are now on Lavender Hill. Take out your A-Z (indispensable) and find the station: Lavender Hill, St. John's Road leading onto Northcote Road and Battersea Rise are all worth an exploration.
This is 'Nappy Valley', an eccentric mixed-wealth area that betrays the stuttering process of gentrification that has taken over London south of the river. One highlight is eating on Battersea Rise - wonder up for a choice of generally good food from all corners of the world, but notice the art deco exterior of Arding and Hobbs, the department store on the corner of St. John's Road and Lavender Hill. Wander up Lavender Hill for a few minutes to find the 'Wrap Factory' for good healthy food. A good 10 minute walk up the hill will take you to the BAC, a great theatre. This area has a number of good bars pubs and cafe's to be explored as well.
Now find the bus stops just outside Arding and Hobbs on St. John's Hill. You're waiting to get on the number 37 or 35 buses, both which take nearly the same route. This bus will take you in a few minutes to some of the best places in south London.
Once you are on try and get a window seat. The bus will turn right up Battersea Rise, past the great restaurants, bars and on to Clapham Common. The bus route will take you through the common, giving you a taste of the informal pleasures the green space has to offer. Get off the bus to enjoy a walk around the park if you like. The bus route continues on to Clapham Common tube station, past some beautiful private houses, hotels (in case you want to stay on!) and large parkside mansions. Once you spot the tube station jump off (or ask the driver to let you know). Again, it is immediately obvious that this area is packed with good food. My recommendation would be the tapas bar Carmen. This area is also home to the Clapham Picture House - a brilliant cinema that shows some really excellent mainstream and arthouse movies.
The poverty/riches clash common to south London is as obvious here as ever, and a bit of people watching will reveal this. Once you have exhausted this area, carry on on the bus route on to Brixton. Brixton is an area with a mixed reputation: traditionally an impoverished, black community, and the site of famous riots, it houses the Brick Lane market - an experience of its own, and some beautiful, if neglected, architecture. It is also the site of a recent 6 month experiment for a more relaxed attitude regarding cannabis. The police did not arrest people they saw smoking.
The Ritzy cinema has as good a reputation as Clapham Picture House (check Time Out for listings before you set out), and opposite, a converted church has become the club Mass. The more chilled out Bug Bar is set in what was the vaults of the church, and with its sofas and decor is a relaxing place to hang out. The drinks are good and reasonably priced for London. The connected restaurant Bah Humbug is really excellent. It has a great vegetarian menu, but solid meat and fish options as well. I always have the Mock Duck, which is as delicious as the real thing.
Other notable establishments are the Dogstar and the Brixton Academy, a great venue, largely because it has a sloping floor meaning you can see the stage even from the back. Sticky floors and watered down drinks come as standard here as in most London music venues. Check listings to see whose playing. When you're done, you can hop on the Victoria Line at Brixton station and be back in the centre in 10 minutes, but you will have seen another London.