Probably testing and level design for me.
To make sure I have no bugs, I test test test. This means playing the game over and over, and eventually getting really good at the game. I then struggle to make balanced levels because I feel they're too easy, so make them harder. I then introduce a friend to the game, and they struggle like crazy. They pass the controls over to me, and I do it first time.
I'm approaching 50 levels on my current game, with another 50 to go. Really starting to struggle to make anything I consider fun. I wish I had another person to work with.