From the Super Mario Wiki
Wuhu Island is a large, tropical, tourist-filled island, inhabited by Miis that appears in some popular Nintendo games, including Wii Fit, Wii Fit Plus, Wii Sports Resort, and Pilotwings Resort. It also appears in Mario Kart 7.
Wuhu Town is the major settlement on the island, and is home to 20 or more office and residential buildings, a bowling alley, and a basketball court.
The town has a main square with a fountain, and two main roads leading out of the town. There is also a tall building on a grassy elevated area. The pier is where a boat normally docks, and is inaccessible.
Appearances in the Mario series
Mario Kart 7 has two tracks the player rides on Wuhu. The tracks include Wuhu Loop (Wuhu Island Loop), which has the drivers race around the entire island and Maka Wuhu (Wuhu Mountain Loop), which has the drivers race around and inside Maka Wuhu, the volcano on the island. A Battle Stage, Wuhu Town, is also present, which allows the drivers to battle all around the town area of the island. However, it is not possible to drive over to the separate tiny village known as New Town. The Island Loop track takes place in the day, and the Maka Wuhu takes place in the evening, while Wuhu Town takes place at night.
Wuhu Island appears as both a stage in of itself and as part of the background of the Pilotwings stage in Super Smash Bros. for Wii U.
The Wuhu Island stage itself is a touring stage similar to Delfino Plaza in Super Smash Bros. Brawl. The platform where players fight that travels around the island often stops at various locations on the island such as the starting line of the power-cruising race course, the mountain summit, the fountain in Wuhu Town, the suspension bridge, the sword-fighting arena, or a moving boat.
In the Pilotwings stage, the yellow biplane on which the second part of the battle takes place flies around Wuhu Island. Rings from Pilotwings Resort (of which the setting is also Wuhu Island) are scattered all over the island, through which the plane flies before returning to the original Pilotwings area.
Songs in My Music
Mario references in other games
In Wii Fit, 8-bit sprites of Mario and Luigi can be spotted around Wuhu while playing the jogging minigame. On a few occasions, a Goomba, Koopa, Mushroom or Star sprite may also appear along the side of the trail.
In Wii Sports Resort, while playing the Island Flyover event, one can hear the Super Mario Bros. theme by flying near the Hillside Cabins area. The name of a ship in this game, the Queen Peach, could be based on Princess Peach.
Similar to Wii Sports Resort, in Pilotwings Resort, while flying next to New Town at night, the Super Mario Bros. theme can be heard. The nighttime description of the town states that one of the dwellers inside is playing the game on the classic NES.