The quest about the boat in the tree needs to wait until after you have the battle tower (same with entering the quarry tower in Broken Valley), so you can still do it. Turgoyn has moved up beside the teleporter, though you may want to read the book yourself, rather than complete the quest (and just don't talk to Turgoyn again, if you don't want to annoy him). You can no longer complete the quest related to the wyvern's nest.
The warning in-game from Sassan before entering the summoning chamber (starting the sequence where you take over the battle tower) is the last chance you have to finish any other quests in Broken Valley or Sentinel Island, trade with merchants, explore most areas, etc.
Outside on Sentinel Island there is some random loot in wyvern nests, as well as something more valuable...
As you move away from the island the poison fog thins out, and you can get down to water level; there is one thing of note to find away from the island, in the direction of the fjords...
the raft Michael was building on the beach in the shipwreck camp; reading a diary you can find on it gives you an achievement.
