Levelling secondary class with no more quests?

[Niwashi]

1. Hunting log. Separate for every class.
2. Guildhests. Every class gets first completion bonus.
3. Leves.
4. Fates.
5. Challenge log.

These are pretty much it for reaching level 15 (if you're out of side-quests). To continue leveling after 15, dungeons will be your best XP source. (These others remain useful secondary sources, though.)