After you go through Belome Temple and defeat Belome once more, you arrive in Monstro Town.
Get the Treasure Room Key
Before you do anything else in Monstro Town, go southeast until you see a key on a ledge. Go into the door below that ledge (if you find a locked door, go into the door to the left of that locked door), then talk to the spiky blue Thwomp at least seven times. Then go outside and you will find the key on the ground. Take it and go northwest, and jump on the spring to return to Belome Temple.
Go back through the temple and go up the elevator, and hit the face on the wall northwest of the elevator. If the message says "Mmm! I'm so hungry", go southwest to the next room, then come back to the face on the wall and try again. When you get the "Sorry, I'm not accepting visitors" message, go down the elevator and follow the path to reach the treasure room. Check on the Belome statue to unlock it with the Temple Key that you got in Monstro Town, then collect all the treasures in the room.
After that, go back to the face on the wall and repeat the process from before until you get the "Mmm! I'm so hungry" message. Then go down the elevator and follow the path to return to Monstro Town.
Explore Monstro Town
In the first door of Monstro Town, just east of the Welcome sign next to the spring, you can talk to MonsterMama to learn about their famous star. Go upstairs and you can talk to the treasure box to find out how many hidden treasure chests you haven't found yet.
Then talk to the star on the shelf, then go downstairs and talk to MonsterMama again. After the cutscene, the Sky Troopas will be waiting for you at the cliff beyond the desert, but before you go there, you can explore the rest of Monstro Town.
If you have the Shiny Stone from Moleville, you can unlock the sealed door to the left of the item shop, but beware that it leads to a tough boss battle that you're probably not ready for (but you can say no to the boss if you're not ready, and come back to try again later). The boss beyond the door is Culex, who is inspired by (but not in any) Final Fantasy games. To get the Shiny Stone, you need to buy the Fireworks from a mole guy in a house in Moleville, then go to the girl who is running a "Pur-tend Store" outside in Moleville, and give her the Firework to get the Shiny Stone.
In the house east of MonsterMama's house, the mouse next to the Thwomp will tell you your record for Super Jumps, and will give you a prize if you have done more than 30 Super Jumps.
In the item shop, there will be a cutscene, and then you can buy regular Mushrooms from the Triplets (but these mushrooms will turn the user into a mushroom, and you can't give them to Mushroom Boy in Seaside Town), and new equipment for Bowser from the Goomba, as well as healing items.
Next to the item shop, there is a bed with a note from the 3 Musty Fears that says you are free to sleep in the bed. You can turn off the mushroom light in the corner to sleep in the bed. If you do, there will be a cutscene where you meet Greaper, Big Boo, and Dry Bones. They will hide flags in the world, then they'll come back and tell you clues to where they are hidden. The flags are invisible, so if you think you found the right place, just press the button to check that area until you find it. Greaper's clue is , Dry Bones's clue is , and Big Boo's clue is . The actual locations of the flags are: , , and . After you find all the flags, sleep in the bed in Monstro Town again, and after you wake up,
In the southeast part of the town, there is a hole in the ground that you can fall into to use a hidden Save Block. Northeast of that, there is a door that leads back to the world map, allowing you to easily leave and come back to Monstro Town. In the exit room, you can stand to the left of the bush and jump to reveal a hidden chest that contains a Frog Coin.
If you go up the stairs next to that exit door, you can go through the door at the top and there will be a cutscene in a dojo. After the cutscene, you can talk to Jagger to battle him. He is strong against Fire and Jump attacks. If you defeat him, Jinx returns, and you can battle him. If you defeat him, he'll challenge you to a rematch, and if you defeat him again, he challenges you to one last rematch. The rematches will be very difficult at this point, so you might want to come back later.
After you are done in Monstro Town, if you don't mind spending 100 coins, it is recommended to go back to the western corner of the town and jump on the spring to return to Belome Temple. Make your way back through the temple, and pay 100 coins to use the spring to return to the desert. From there you can go north to the tall cliff where the Sky Troopas are now floating in the air. (If you would rather not spend the coins, you can use the Monstro Town exit to go to the world map, then go from there to Land's End, and go all the way through the obstacles in Land's End, including the ant whirlpools, to reach the cliff.) You can jump from troopa to troopa to reach the top of the cliff. From there, go northeast and you'll be back in the world map, where you can proceed to Bean Valley.