For writing, get the Egyptian incense and put it near your sim - it will slow down the rate of fun decay. Also, it helps to stick with writing children's books - they aren't a whole lot of Simoleans each, but you can dash off two a day with a break in between to get that mood back up, and on a per page basis, they're actually outstanding money makers.
On topic: like Wai, I put a sim through the cooking meatgrinder first thing, to get the fridge. I'll make a sim a scientist to get some time with the upgrade gun, filling my house with objects that have positive mood let auras. But hands down, my favorite, especially for established families, is gardening. There are no hours, and if you have multiple sims in the house in the gardening profession they all get credit from the same harvest. It's ridiculously fast to progress, too - a stand of money trees can get you to level 10 in one day.