The chest can be brought down with the gale boomerang,
As for the monkey (you may have done some of this, i just took this part from the walkthrough I'm writing for this game):
In the room where all the monkeys gather make your way up to the eastern door, cut the vine walls along the way and slingshot the spiders and climb in some vines to enter the door.
You will see some ledges (one has a big rock on) and some red bomb plants, equip your boomerang and target the bomb monster, then the rock wall. Climb the ledge and kill any monsters, walk to the end of the ledge and gale another bomb to you, blast the rock to find a monkey. Cross the bridge and gale another bomb to take out the eating plant and open the chest to get the small key. Go back to tile monster room and use the key.