Kew Gardens in London is quite extensive and lovely to visit any time of year. They feature wonderful glasshouses. The Waterlily House was particularly quaint.
We visited Kew Palace and the Kitchens as well as Queen Charlotte's Cottage for a historical view.
The Xstrata Treetop Walkway was very cool where one can stroll among the soaring trees and just glimpse the modern London skyline.
A favorite spot for us was the rock garden housing water features, rocks, and all manner of succulents.
Kew Gardens can be easily reached via the London Underground. Plan to make this an outing for the entire day or more. We spent the day there and still did not cover everything.