Balloon Berries are items encountered in the Haunted Towers. These items possess a special ability: if Luigi vacuums the berries, it will give him the power to fly around, by sucking in or blasting out air.
The power from the Balloon Berries will only last until either Luigi touches something spiky, something hot or until he sucks up the berry, otherwise it'll stay stuck to the Poltergust 5000. Another variant of the berry is the Golden Balloon Berry, which is similar but generates a trail of coins. If Luigi does collect all the coins, he is rewarded with a chest that will contain either treasure, a gem or in the case of Treacherous Mansion, a Boo.