Golden Melons are a wearable milestone that have golden melons spinning and floating around you, and also have lights on them.
There are also Golden Melons that are hidden around Ball Race maps (3 for each map). To find them you have to get near where the Golden Melons are and they will reveal themselves. Getting the Golden Melon achievements will not unlock the Golden Melons milestone, as milestones are unlocked with EXP. However, achievements do give you some EXP.