Anything. Everything. Crafting and gathering exp is so easy to come by now, they may as well skip leveling altogether.

Weekly custom deliveries. Massive exp, limited to 12 turn ins per week. Not difficult, but not strictly trivial, depending on item/level range.
Beast tribes. Good exp after 50. Dunno if Ixali exp is decent, but it's there. Trivial difficulty. Gatherers can do Namazu quests after 60.
Ishgard Restoration. Crazy exp. Collectibles, so, about as difficult as the custom deliveries. Does require some upfront gathering, but the special items have some high yields to make this not too tedious.

Leves are probably still good, but personally, haven't touched them in a long while, simply due to naturally leveling doing any other easier thing. But, maybe that all depends on how much you want to pay attention to what you're doing.