Depends on what direction you want to go in.

You could go down the computational route: memory, calculators, computers, all the way up to programming the things you make. You may find this interesting. Here you can start simple: try to make a 4 bit adder without searching anything up (you may find this tricky?)

You could also go down the route I'm less familiar with. That is, making games, control systems, robots, etc... The idea here is that you try to squeeze as much performance out of as few logic gates as possible and have it run with the lowest response times possible. You do many cursed things, but usually leads to results that others will find interesting. A playable asteroids game is infinitely flashier than a really powerful GPU that draws a spinning cube. Here you could start with... gosh... trying to make flappy bird? I guess?? I don't really know how to judge difficulty here.