Greenwich is one of the most rewarding half-days out in London. Start at the Cutty Sark — the last surviving Victorian tea clipper, now a museum ship in dry dock near the river. Walk up through the Old Royal Naval College (free to enter, the painted hall is extraordinary) and climb the hill to the Royal Observatory, where you can straddle the Prime Meridian — the Greenwich Meridian, zero degrees longitude by international convention since 1884.
The views from the top of the hill across the park and the Thames, with Canary Wharf rising behind, are among the best in London. The National Maritime Museum at the foot of the hill is free and excellent — the story of Britain and the sea, told through charts, instruments, paintings, and ships.
Getting there: Fastest is the Docklands Light Railway to Cutty Sark station. Alternatively, take the Thames Clipper river bus from central London — a pleasant approach.