Hehe, I think I was lvl. 19 too JUST before I battled Laiken and claimed MY battletower. There isn't too much to do on Sentinal Island though, and don't bother trying to find this Spellbook high up in some strange rock/tree. You can pick it up later!
O, but I have find many quests to solve here on Sentinel Islands and also hidden secrets and a beautiful sky and I go also for a little swimming héhé ... and find hidden treasures when go on places only accessible via swimming

... and now, before enter the Battle Tower and deal with Laiken, I go back to Brocken Valley and Orobas Fjords because now I'm stronger than before (on that moment L 13) I go to Orobas Fjords ... and there is something to do in the "Mines" ...
And yes, I was thinking about that ship in that big tree ... I have the feeling I know how to solve that quest Willem
...
I take my time to solve quests, kill enemies, explore area's, go for trading, study weapons and armour, skills and so on ....

...
that's my way to play this RPG
For me this game has a loooong play time!! I evaluate that I do at least more that 100 hours playing before I end up in the finale battle.