I love discovering new places in London, even ones that have been under my nose all along. Have you ever been to the Barbican? It's a development in London like no other, it's an art centre, people live there, some have their offices there - it's the closest thing London has to a concrete oasis. It's all concrete and plants, and really comes alive in the summer.
In the winter however, there is one place you can go to really get a feel for what the Barbican is all about, the Conservatory. It's a bit like a grungy Kew Gardens in there, and like the Barbican itself, it's easy to get lost in.
It's a fun afternoon out, it's free, and it gives you a chance to feel like you're in some futuristic post-armageddon city, where the plants have taken over. It's a bit like a dream land and definitely worth a visit.