Unlockables

In the Party Shop, you can buy the following things with the coins you cash in from Stars at the end of Party Mode.

Dry Bones (2,000 Coins)

Boom Boom (2,000 Coins)

Pom Pom (2,000 Coins)

Bowser Jr. (3,000 Coins)

Pianta (3,000 Coins)

Noki (3,000 Coins)

Luma/Polari (3,000 Coins)

Brutal Difficult for CPUS (1,500 Coins)

How to Unlock Alternate Boards

Eternal Star (Mario Party) - 10,000 Coins in Party Shop

Waluigi's Island (Mario Party 3) - Win in Party Mode online/offline as Waluigi 3 times.

Bowser's Gnarly Party (Mario Party 3) - Play All Other Boards (not unlockables) 

Koopa's Seaside Soiree (Mario Party 4) - 5,000 Coins in Party Shop

Faire Square (Mario Party 6) - 5,000 Coins in Party Shop

Pyramid Park (Mario Party 7) - 5,000 Coins in Party Shop

King Boo's Haunted Hideaway (Mario Party 8) - 5,000 Coins in Party Shop