To be honest, one of the primary reasons I wanted to go back to London was to visit the Royal Botanic Gardens, Kew. In my last visit to England, we saw a documentary on the BBC about how Kew Gardens were instrumental in creating the British Empire. That cemented my desire to see the place for myself.
I'm sure it's even more amazing in the warmer months, but there was more than enough to occupy us for 5 hours as it was! And we didn't even get to see everything. My husband said: "It's so easy to find things here. And the walks are really pleasant." I'm guessing that those 2 things were not, in fact, accidents and are indeed just good design. Kew is certainly worth a trip, in our opinion!
The greenhouses are really beautiful buildings. Overall, the plants inside didn't personally knock my socks off, but then again I'm living in the 21st century where air travel makes it easy for me to visit exotic locales and I grew up in a tropical climate. I can imagine, though, how amazed Victorians would be at their contents. All the exotic palms from all over the empire! Chocolate trees! Coffee trees!
In addition, the Order Beds (at least the ones with stuff above ground) were fascinating. Each bed contains plants from the same family & then those beds are grouped by order. The similarities and differences between plant "cousins" was really interesting. I hope to go back to explore this a bit more at some point.
My favorite bit might have been the Davies Alpine House and the rock garden near it. Perhaps because they both contained so many unusual flowering plants (who isn't a sucker for flowers?). I love how they were all grouped too!