Go through the battle tower, talk to everyone and loot the place (with your trainer you can unlearn your skills if you wish, and have the gold, or raise the level cap of some), then head back outside on Sentinel Island to finish off the remaining opponents. You can now get to the boat in the tree to retrieve the treasure that someone wanted (now moved up beside the only remaining teleporter outside), though you may want to use it yourself rather than complete that quest. There are also a couple other things of note to find, as well as a few chests in wyvern nests with random loot.
After that, head to the fjords and start exploring. The mountain path and beaches would be the easiest to do first, and there are several caves you can reach on foot or by flying. There is a barrier blocking off the start of the fjords, which you will need to disable to continue farther.
