After you meet Gaz in Rose Town and hear rumors that Geno went into the Forest Maze, you go to investigate.
Explore the Forest
Pick up the mushroom in the corner, and before you leave this area, go to the west corner and jump to find an invisible chest that contains a Croaka Cola/KeroKeroCola. Here is a picture if you are having trouble finding it: second forest maze invisible chest 1. Go northwest to the next area.
Pick up the mushroom, then go northwest to find a hollow tree stump with a Wiggler walking around it. If you're quick, you can run up to the Wiggler to battle it, but this is optional. It will cast "Vigor Up" when attacked, which increases its attack power. After you defeat it, Wigglers will keep coming out of the tree stump, so you can use this to get more EXP if you want. When you're ready to move on, jump onto the tree stump and go down.
In this area, approach mushrooms slowly, because some of them will turn into monsters. Go to the east side of the area and jump on the spring to go to the next area.
Go west of the tree stump and jump up and down while moving around, until you find an invisible chest that contains a Frog Coin. Here is a picture if you are having trouble finding it: second forest maze invisible chest 2.
Then go northeast, battle or avoid the Buzzer (it may chase you when you get close), and use the north exit to reach the next area.
There are seven tree stumps in this area. Most of them lead to an empty area, but three of them lead to an area with a hidden treasure chest. However, one of the hidden treasure chests is empty. To get the two non-empty invisible chests, go down the easternmost tree stump, then stand in the southwestern circle of light and jump there to find a hidden treasure chest that contains a Fire Flower. Then go down the northwestern tree stump, and go to the northwest corner, avoid touching the Wiggler, and stand behind it, and jump to find an invisible chest that contains a Croaka Cola/KeroKeroCola. (The empty treasure chest is down the middle of the southwest line of tree stumps, and can be found in the corner north of the spring.)
Before you leave, touch or jump on the Wiggler, and go northwest from the tree stump to the new area that opened up. Go down the tree stump that you find there.
Go east, then northeast, and the path will go south. Go to the south end of that path and go southeast to reach the next area.
To the east, the path splits in a north and south path. The north path has two fake mushrooms that will attack, while the south path is much more narrow and has a hidden mushroom that will rush out from behind the mushroom pile. Whichever way you choose, go to the east side of the area and jump on the spring.
Go northeast and use the save block, then go northeast, but before you leave the area, pick up the mushroom, then go into the eastern corner and jump to find a hidden chest that contains Red Essence. When used in battle, Red Essence prevents the enemy from attacking you for three turns.
Go northeast and you will enter the maze. You will see Geno, but before you follow him, you can find the area that the person in Rose Town told you about. To get to the hidden area, go northwest, then southwest, then southwest again, then northwest. Go down the tree stump that you find, and open the chests to get one or more Fire Flowers, Frog Coins, and Mushrooms.
Back in the forest maze, you can go southwest to return to the entrance, then save if you wish, then go northeast to re-enter the maze. This time, go southeast, northeast, northeast again, southeast, northeast, and finally northwest. After a cutscene, you have to battle Bowyer.