I've enjoyed playing Sims that live without careers or traditional skills. Exploring the catacombs was a waste of time for a livable income, but other methods have worked much better. Kleptomaniac was my favorite, I think, even though you can only steal a few times each night. I laughed a lot at the goofy things my Sim stole. Seriously, there's a laptop in the room and he takes the drapes (they had very nice drapes, though!). It beats stealing the lights outside people's houses. It's probably one of the more interactive ways to make cash in the game.
What way above has worked best for you and what will you try next?