Kew Gardens is 132 hectares of botanical garden on the south bank of the Thames, containing over 50,000 plant species and several remarkable Victorian glasshouses. The Palm House, a vast iron and glass structure completed in 1848, is an experience in itself — the dense humid heat hits you as you step inside, and you walk through banana trees, cacao plants, and palms reaching the full height of the building.
The Temperate House, recently restored and twice the size of the Palm House, holds the world's largest collection of living plants under glass. The Japanese garden, the rock garden, and the Treetop Walkway (18 metres above the forest floor) provide completely different experiences within the same site.
Allow a full half-day at minimum; a full day is easily spent.
Nearest tube: Kew Gardens (District Line).