I'll join the recommendation to run GC hand-ins and Ixali quests.
The GC hand-ins especially at the lower levels can be quite easy/cheap to make, give a lot of experience, and there's always the bonus of GC seals.
For the Ixali, think of it as a long term investment. Once you get high enough in the ranks with them, you can exchange their unique beast-tribe currency for ranks 1-3 gathering and crafting materia, and the latest rank gives you rank 4 materia. Kind of getting ahead of things here, but if you'll want to invest seriously in end-game crafting, those are all things you'll be glad you have relatively easy access to.

Other than that, it's leves. There're a lot of good guides out there about what leve is the best as far as cost-exp ratios go, but I personally like to run them all at least once. Get that first-time-crafting bonus while you're at it.