I like to do origami. You just need paper, perhaps a cutter and ruler to get square paper from A4/letter. It doesn't usually require a lot of thought which can be a deterrant to sustained practice. You can either fold from diagrams found in books (thousands are available, look around in zlib), or if you can't have a screen in front of you without going to 4shit, you can do tessellations, where you essentially just let your own creativity guide you. If you like to go deeper, there is plenty of interesting mathematics to it.