If this weren't enough, think about how good you will feel helping to keep the environment clean. Beware that there are sometimes restrictions, such as only being able to travel after the morning rush hour. In London, buy a Travelcard based on zones. Travelcards can be purchased for one day, seven days, or a month. An LT Card, which costs a little more, doesn't have time restrictions.
Most tourist destinations are in zone one.
If you take a trip into any other zone, you just pay the difference. Buy them at visitor's center and some Tube stations. Purchase discounted books of 10 tickets, called carnets. Both are sold according to zones. They are available at tourist offices, hotels, and from travel agents.