I guess the hard part is to find something complex enough that you'll get motivated and simple enough that you can get done on your own in the free time you have. And software is pretty good at teaching you how complex simple things can be.
Challenging is probably better choice for a word then complexity. Anyway, there are a few different kinds of complexity.
One comes from simply the problem at hand being large / complex and that's something that might be really hard to come up with.
However, it can also be challenging because of unfamiliar tech stack. Be it a new language, new framework, etc. Going too far in this direction can lead to projects being too hard to complete so you want to balance it with familiar technologies as well. For example, if you are familiar with Python and Javascript, picking Elexir and CoffeeScript might be too much. However, Python and CoffeeScript would make a good combination. You get the idea.
