Let's face it: the London tube lines can be really confusing at first. Knowing which platform to get on and which stop is what you need can be excruciating. Although Apple updated its Maps feature last year to include public transport, for those that don't have Apple products or don't know what to see in London, here is a quick intro to some (but not limited to) of the awesome things you can do, broken down by each tube line.
Northern Line
Running through the heart of London, chances are you'll have to take the Northern line for a lot of places you want to visit. Camden Town is a hugely popular tourist attraction, with lots of opportunities to buy souvenirs. I visited Nemesis Tattoo in Camden on a whim and left two hours later with my first piece of ink.
Platform 9 3/4 is located at the King's Cross St. Pancras which is also on the Northern Line! Note that at King's Cross you can change for the Victoria, Piccadilly, Circle, Metropolitan, and Hammersmith & City lines. Also, don't forget to visit the famous London Eye, which is right off the Waterloo exit (intersected with the Bakerloo and Jubilee lines). Be sure to check out the London Dungeon while you're in the area for a dark history of London's past.
Bakerloo Line
One of the coolest things you can do on the Bakerloo line is visit Baker Street where the world's most famous detective lives - Sherlock Holmes. Also located at the Baker Street stop is Madame Tussaud's wax museum. Baker Street station is also located on the Metropolitan, Circle, and Hammersmith & City lines.
While you're at it, stop by Oxford Circus to get some shopping done (also on the Central and Victoria lines), and Piccadilly Circus to check out West End (also on the Piccadilly line). If you're feeling historical, hop off at Lambeth North and visit the Imperial War Museum.
Central Line
By far, the best thing you can do on the Central Line is visit St. Paul's Cathedral. You can walk the stairs to the top of the building which gives a captivating look around London. Also check out the British Museum at Holborn, which you can also access via the Victoria Line.
Circle Line
Liverpool Street is a popular place to leave from if you're looking to take the train out of London to visit other areas. It can be accessed from the Central, Metropolitan, and Hammersmith & City lines also. If you want to embrace your medieval side, you can stop at Temple (also located on the District line) and visit Temple Church.
District Line
If you do nothing else during your time in London, Christ on a cracker, go to Westminster. Conveniently located along the Jubilee and Circle lines also, there are SO many cool things to see from there. Although you can't walk down the street and knock on the door, you can visit Downing Street, where the home and office of Prime Minister David Cameron is located. A short walk away is also where the Houses of Parliament are located (and for the record, Big Ben is the name of the bell. The building is named the Elizabeth Tower, often called clock tower.)
Hammersmith & City Line
Located at Barbrican (also on Metropolitan and Circle lines) is the renowned Barbrican Theatre. Go there for some really cool performances! If you're feeling royal stop by Great Portland Street (on the Circle and Metropolitan lines also) and visit Regent's Park, perhaps on a (rare) sunny day where you can bring your lunch with you.
Jubilee Line
Have a quick stop at St. John's Wood to check out the famous Abbey Road Studios or take a journey to the outskirts of Zone 2 for a concert at the O2 at the North Greenwich exit. Catch the Waterloo station (along the Northern and Bakerloo lines as well) to the South Bank of the River Thames and get a boat tour across the city of London.
Metropolitan Line
Wembley Park is the nearest tube station for Wembley Stadium (also on the Jubilee line) if you're looking to get your EU football fix (note: Wembley Park is Zone 4, so if you're only covered for the main zones (usually 1-3), you'll have to pay a small fee on your travel card to get there and back.)
Piccadilly Line
One of the busiest tube lines in London, and with good reason. If you're looking to travel out of Heathrow and intend on taking the tube to get there, Piccadilly line is your only option (note: Heathrow is in Zone 6.) Be sure to stop at Hyde Park Corner to spend time at the lovely Hyde Park. If you fancy EU football that's closer to Central London, hit up the Arsenal stop to see a game at Emirates Stadium.
Victoria Line
Yet another London tube with so much to do and see. If you're a music nerd like I am and want to see home history, take the Vauxhall stop and walk to the Battersea Power Station, infamously known for being the building to grace the cover of Pink Floyd's album Animals. Take a stop at the Victoria station (also on District and Circle lines) and visit none other than Buckingham Palace. Be sure to stop by midday and witness the Changing of the Guards!
Waterloo & City Line
Waterloo & City is a line you won't really find yourself using since it only crosses between two tube stops - Bank and Waterloo. It's a quick way to go from one end of London to another, and runs right under the River Thames. While you're at Bank, though, take a quick walk over the Shakespeare's Globe Theatre! You can access it also on the Central, Northern, District, and Circle lines.
I know a lot of tube lines intersect, but that's the beauty of London. Their map is really elaborate and those intersections keep you from having to jump from line to line all day.
Now that you know some of the basic places to stop on every line, you're ready to begin your journey through the amazing city that is London. Have a blast!