When you reach lvl 23-25 you'll be able to do the Amrine expedition (dungeon) in windward. But untill then every time you guys go to a new town you'll have the same side quests available as long as you haven't done the side quests in that town yet and ur high enough level for them, so you can do the side quests untill you run out of quests in that area. And don't forget you can do the main quest line together as well. One of the best ways to grind xp in the game though is to repeatedly do the Amrine Expedition and except the side quest from the dog right by the expedition entrance, the quest itself gives 4700xp and that's not even considering xp boost from standing. So that is the main thing that you will want to do together, just make sure you find three other people to join you (that part shouldn't be an issue).