Magma Mountain - All you have to do is beat the first 6 stages. In these 6 stages, you have to buy things in the shop. After that, you can play Magma Mountain.
Eternal Star - Get 100 stars in Magma Mountain, then a special event will appear. After that, Eternal Star will appear. By the way, after beating Eternal Star, you will see the ending, and some more, special things can be bought in the shops.
To access these mini-games, you must complete the following:
Bumper Ball Maze 1 - You must reach the goal of Mini-Game Island and defeat Toad in a game of "Slot Car Derby 2."
Bumper Ball Maze 2 - You must clear all 50 Mini-Games on Mini-Game Island and then go talk to Toad at the Goal.
Bumper Ball Maze 3 - Once you have set new records on Bumper Ball Mazes 1 & 2, Bumper Ball Maze 3 will become selectable.
Go to Minigame Stadium in the option house. Set the computers on hard, use handicaps and set 50 coins for each player, and set it for 30 turns. When you are near the end of the game turn the computer players to human players so that you can get all the coins they have raked in. Every time your do this, you usually get 800-1000 coins.
Every time players complete a game, the stars and coins from each human player are put into the Mushroom Bank. However, if computer players are filling in, those coins and stars are lost. An easy trick to accumulate lots of stars and coins is to switch all players to human in the last round of the game. (You can do so by pausing and accessing the options menu.) Complete the board and you will be able to keep all of the stars and coins.
After you complete Mario Party by earning 100 stars and by finishing the "Eternal Star" board, "No Boo" will become available in the Mushroom Shop.
After you complete Mario Party by earning 100 stars and by finishing the "Eternal Star" board, "No Koopa" will also become available in the Mushroom Shop.
You can get a song test by buying the record at the mushroom shop. For additional songs, you return to the sound test after beating the "Eternal Star" board. You will find that about 20 more songs have been added.
The title screen will change to whoever you beat the game with. ie: If you beat the game with Mario, then the Title Screen will change into Mario's.
You can purchase the ending sequence of the game from the Item Shop, once you've earned 100 stars and have completed the "Eternal Star" board. Once purchased, you can watch the ending at any time from the Option House.