For a single day, a Zone 1-2 Travelcard will set you back about £15.20. That’s the price of a fancy sandwich and a cinema ticket. But it buys you unlimited Tube, bus, tram, DLR, London Overground, and even most National Rail trains in those zones.
Think of it as a magic rectangle. Tap it on a reader, and the city opens up. No fiddling with change, no panicking about the right fare.