Most of the multiplayer modes have to be unlocked before you can play them, including the target ranges, the 2D Super Sprint clone and the 3D racing game. Here's how you can get them:

King of the Hill
Location: Cerulean Holding Room
Type: Totem Pole
Character: Any
Requirements: None

Tunnels Stage
Location: Rith Essa Waterfalls
Type: Totem Pole
Character: Vela
Requirements: None

Rith Essa Mine Stage
Location: Walkway Station
Type: Totem Pole
Character: Any
Requirements: Powered (jetpacks)

Space Station Stage
Location: Space Station Basement
Type: Totem Pole
Character: Any
Requirements: None

Goldwood Target Range
Location: Goldwood
Type: Floyd Mission Award
Character: Floyd
Requirements: Get Gold on Goldwood Floyd Mission

Rith Essa Target Range
Location: Eschebone
Type: Floyd Mission Award
Character: Floyd
Requirements: Get Gold on Eschebone Floyd Mission

Jeff & Barry Arcade Racing
Location: Ichor Arcade
Type: Arcade Score Reward
Character: Any
Requirements: Come in first on Jeff & Barry Arcade Racing

Jeff & Barry Arcade Racing II
Location: Ichor Arcade
Type: Arcade Score Reward
Character: Any
Requirements: Come in first on Jeff & Barry Arcade Racing II

Greenwood Village Race Track
Location: Ichor Arcade
Type: Arcade Score Reward
Character: Any
Requirements: Break the top records on both Jeff & Barry arcade machines.

Mizar 3D Racer
Location: Mizar's Palace
Type: Race Award
Character: Any
Requirements: Come in first in the Mizar races.