The first area is found in level 1, the forest. After a few enemies, you'll come across a large tree stump. Have the Smurf jump on the floating leaf and then jump to the tree stump. Stand in the center of the tree stump and press down. The Smurf will enter a secret room containing 3 leaves and a berry. Jump on the springboard to exit the room.

The second bonus room is located about half way through the forest stage. Use the bee to jump on the tree stump and press down to enter the second hidden room. This room is basically a carbon copy of room #1 except for the items it contains. Collect the star, 2 leaves, and berry before leaving via the springboard.

Hidden room 3 is located in stage 3 towards the beginning. Jump over the two frogs and you'll come across an area which contains three vines leading to the top of the screen. Climb the first vine of the three and you'll climb off the screen into a hidden area. This area contains 2 leaves and a berry.

Hidden room 4 is also located in stage 3. About half way through the stage, you'll come across another group of vines leading to the top of the screen. Climb the left vine off the screen and you'll enter hidden room 4. This room contains two leaves and a berry.

Hidden room 5 is the last of the hidden rooms to my knowledge. In stage 6, the mountains, walk to the right at the beginning of the stage. You'll come across a mountain which leads off the screen. Walk down this slope to enter hidden room 5. This room containes a berry and two leaves. To leave, jump off the right side of the screen.