I go to London Zoo every year, I don't even have children to take with me, I just go with my partner. Animals are for grown-ups, too!!
The new Rainforest experience bit is fantastic. It's just very well done, you are in the room with the animals, no cages, no vivariums. Tiny little monkies are playing a few feet infront of you. I probably spent about an hour alone in this room spotting and watching the wildlife. That may also be because I spotted one of my favorite animals, a big hairy sloth :D
The butterfly house is great, too. A bit of an ayesore on the outside with this crazy inflatable entrance, but inside it's beautiful. All these different kinds of butterflies around you, and then when you look a bit closer, you notice there's hundreds more. The sleepy moths are pretty incredible, too.
My other favorite part is the nocturnal animals. Very interesting and often strange-looking creatures.
The food is pretty expensive in the restaurant, so I'd advise bringing a packed lunch with you. But that's really the only negative thing I can think of. It's a great bit of escapism for the day, learning about the animals we share the world with and putting a few things into perspective.