I don't recommend achievement hunting but rather mount collecting. SE adds mounts to things they want people to do, knowing that it won't pressure people to do it who just want to complete quests and do it once.

For example, there is a mount for doing all of the blue mage content, collecting triple triad cards, winning 100 or more of each pvp match, crafting and gathering a lot in the firmament, hunting a lot and ocean fishing a lot. There is a mount for doing beast tribes. Mount collecting takes you through a variety of different content and grinds which take a lot of time if you want something to occupy you after clearing all the content.