RB1 river bus from Greenwich Pier to Westminster runs every 20 minutes, costs same as tube (£2.80 with Oyster). Takes 50 minutes but you see entire city from water.

Best views of Tower Bridge, London Eye, Parliament without paying £30 for tourist cruise. Heated boats in winter, open decks in summer. Pick up fish and chips at Greenwich Market first, eat them on deck.

Note: Thames Clipper prices vary by zone and route length — Some central stretches cost more, but this particular route is the bargain.