I've really got into colouring recently! I was given a colouring book as a present a few months ago, I realised I hadn't done much with those I'd got last Christmas, and the librarian at work gave me some Christmas colouring bookmarks!
I realised that while colouring is quite a simple thing to do, it doesn't have to be mindless and un-creative - there are lots of ways that you can experiment and make your colouring pages unique by adding shading and colour gradients.
I'd coloured in one of the Japanese colouring postcards last year with Tombow brush pens, but was disappointed with how flat the colour was and how streaky the finish was. After reading a bit about colouring techniques, I discovered that I could use pencil on top of the pen and I was delighted with how it turned out - I've used this technique on a few other pictures since.
I've discovered that I particularly like colouring postcards and bookmarks, because they lie flat and are handy to keep beside you for those idle moments. I've actually considered cutting up some of the colouring books for this reason!