Kew is a whole day if you want it, but a six-kilometre walk handles it cleanly if you use the Thames Path to exit. Morning light in the Palm House, the Pagoda for the view, Queen's Garden for the quiet, and then the river home.
Enter at Victoria Gate and go straight for the Palm House. It's the one piece of architecture in London that I think you can genuinely call sublime; Victorian iron and a hundred thousand panes of handmade glass. Take the iron staircase to the balcony — nobody does and the humidity is worse up there, but the view back down is the best possible.
South to the Pagoda, then diagonal north-west past the Temperate House (under restoration on rotation; still vast) to Kew Palace. The Queen's Garden behind it is my favourite quiet corner of the gardens — knot hedges, clipped yews, the world outside shrinks to a dot.
Exit via the river gate and you're on the Thames Path, heading north. Forty minutes of flat, slow water. End at Kew Bridge with the sense of having done a day in a morning.
