GC hand ins are great, but only once-a-day for any given class.

Leves are your best bet up to 50 or so. Simply gathering stuff works well, but not quite as well as Leves.

Once you unlock Collectibles (50 or 51 I forget which) and flight in the various Heavensward zones, forget everything else, that is what you want to be doing. Aim at getting the highest quantity of the lowest collectability rating that you can hand in, for the most exp. IE - Old World Fig gets 60k exp at 240 Collectability, 66k at 400 Collectability, and 72k at 450 Collectability. You actually get more experience handing in three 240 Collectability OWFs than you do handing in two 400 Collectability OWF's. The same principle applies to all the Collectables you gather.

Leves are still good in the 50+ range - but not as good as Collectables. They do supplement Collectables though.