I like colouring in. I don't care that it is meant to be for children because I find it relaxing and satisfying (being artistic but not actually able to draw I like that there are lines to keep within). There is nothing so nice as sharpening pencils and then sitting down to colour while you listen to the radio or even if you are having a picnic in the park (not very likely in London this week I know). Go on, try it, it's pretty fun.
Some lovely colouring books for adults and children are:
The architecture colouring book from the Southbank centre shop.
Also the Arthur Rackham colouring book (I actually did a little dance when I found this), from Amazon and the book depository and others I'm sure.