Fishing. Again, max over many! If you’re serious about fishing, get one toon maxed out in fishing for best money-making results. While leveling, once you’re beyond apprentice, go fishing for Oily Blackmouth, as it sells well since it’s an important mat for alchemists.
Once you’re maxed out, focus on limited availability fish that provide the largest stat boost. In WotLK, Dragonfin Angelfish are the best fish out there, as they can be cooked into food that provides the highest food boost to either strength or agility.
Cooking. Have a toon with maxed cooking. This is relatively easy to do.
Considerably more involved is running the repetitive daily quest to get 100 Dalaran awards to get a Chef’s Hat. Once you have one, though, cooking in volume without it will be a drag. Plus, a hat opens up the potential for buying raw fish to cook for others. You may not find this tolerable if you cook slowly, but once you can cook 3 stacks per minute, time is no longer a serious obstacle.
First Aid. You must be kidding! First Aid is to making money as fish is to ice cream.