I've done much of the simple GPIO stuff. LEDs, buttons, temperature sensors, PIR motion sensor, photosensor. I've also learned (the hard way) about protecting the Pi and backing up the work I've done.
Now I'm about to break into transistors, higher voltage and a more interesting device, e.g., a solinoid-operated water valve.
If any of this confuses you, you should scroll back a few episodes in my blog.