Mini Quests

[Bilgepump]

These are the only ones I do regularly.

Josie McReedy in Port Royal, and Johnny McVane in Tortuga (both bartenders in pubs) will hand out sets of "defeat enemy" type quests. They both draw from the same quest pool, so if you keep asking, you can get the same quest from both of them. The more quests you do, the bigger the reward you get for the next one. For example, they each give me a quest to kill 8 undead gypsies, for 145 gold reward. I go to Padres beach, kill 8 undead gypsies, and collect 290 gold for doing something I would do anyway.

Bartholomew Watkins in Port Royal (he's the guy hiding behind the barrel) will give out three quests to sink different kinds of ships, each one paying 35 gold. But the quests are bugged, and will complete on any ship.
If you log off and back on, he'll give it to you again. That's a quick extra 105 gold for every trip.

Black Mack in Port Royal will give you a quest to win 150 gold at poker. Whenever I got a win-at-poker subquest on the Black Pearl story quest, I'd visit him. Get reward for both from one round of poker.