Small origami structures are a bit more complicated than the bigger ones, mainly because there is not enough space for your fingers to work around. And of course, there are more chances that your detailing could go wrong. Nevertheless, they are actually quite fun to try with so many useful items that you can create.
One of the popular smaller designs is the origami keychain. Quite simple to make and it comes with an array of interesting designs.
Just follow the instructions in the link and you will be able to create any design you prefer.