Sudoku is that for me. Feels exactly like work, but perfectly balanced where is hard enough to be interesting and easy enough to be fun.
I wrote a computer solver for Sudoku and that was enough for me, I feel no need to ever play again ;-)
I play ALOT of Sudoku and the game now for me is optimizing my own thoughts while I play. Moving from a random search and find function, to a structure queries that solve the puzzles quicker and quicker. Sort of like a rubix cube. I focus on beating my best time and making it easier and easier.