No particular order:
ANYTHING engine related on my Saturn. Running gear and whatnot was easy, but that engine was like a solid block of plastic. I literally had to remove a motor mount to change the fanbelt. Swapping out the starter motor was just stupid.
I had a 73 VW Beetle for a while. Fun car. Burnt out the #3 cylinder after a while, so I pulled the old engine, bought a new long block, swapped all the junk over, and installed it. I'd never done it before, and while it wasn't too bad, I had to do it in the backyard, with not much for tools. I used a skateboard for an engine dolly.
So after a couple weeks of weekends I got it done, after exhausting my bank account and driving my wife up the wall. I loved that car. Anyway, the first time out the wiring harness caught fire and the car was totalled.