With 37 Hidden Rooms in World 2 it's no joke that a guide is almost necessary, especially for some of these obscure locations! I recommend you run it on Infantile Mode to skip all those painful jumps. You'll also need bombs, a torch, the wooden whistle (with the gatekeeper's song) and the Glove of Wisdom. Read below for all the secrets!
Everything but the first Hidden Room starts after the rafts, so get to that point and then get your butts ready for the "fun". Only tool you honestly need is the bomb.
Hidden Room 1
After falling off the raft, and moving on to the main area look to your left and you'll see a lone shop.
Hidden Room 2
After getting past the water geyser part, and through the waterfall, look to your left after the checkpoint and jump up the rocks and around the ledge. After getting all the way around chuck a bomb at the wall.
Hidden Room 3
Go ahead and die, or get back to the checkpoint, and head to those hillbillies across from it. Turn right into the forest and navigate the trees until you find a yellow tree to torch (it'll be near a bear). After burning the tree, chuck a bomb to reveal your next hidden room.
Hidden Room 4
Back to the Hillbillies! This time clear them out and go behind them to a ledge of rocks and down to a secret lower tier. Navigate up the river to a jump pad, and up to a yellow bauble. At this yellow bauble chuck a bomb at the wall.
Hidden Room 5
Get to the river with the logs moving down river and various crocodiles. If you look down river you will see a lone geyser. Get to it, and follow the path of the screenshots to find a secret shop!
Hidden Room 6
At this shop (from the previous secret) drop a bomb near that odd log and check out this creepy hidden room!
Hidden Room 7
When leaving the shop chuck a bomb at the wall to your right for a cave of deadly toads.
Hidden Room 8
In the cave of toads follow the screenshots above and throw a wall shown. It's very close to the intersection of the two parts of this secret area.
Hidden Room 9
Right before the river with the floating logs, at the shore you begin on, is a secret area behind the first waterfall. Just walk through the water and you'll see the spot to bomb.
Hidden Room 10
Pretty much the shop with the hillbillies near it, right before the checkpoint.
Hidden Room 11
Get to the waterfall with the logs falling. Near the ledge with the second highest jump-pad will be a secret bomb spot. Your best bet is to get on one of the logs and pose your camera to look through the water. Chuck your bombs occasionally and eventually you'll see a strange area to enter.
Click the waypoint above to jump to Zone 2!