There are items called Capsules in the game, and while they don't change gameplay, they add funny or odd things to certain courses. Each course in the game has two Capsules to unlock. You must play twenty times on each course with both types of tee (regular and long). So, by playing forty total times on each stage, all Capsules can be unlocked. Just be mindful of your tee type!
To unlock the following characters listed, defeat them in Challenge Mode: Anya, Bjorn, Bloom, Dino, Felipe, Fernando, Kate, L.J., Marayuma, Sasha, Sonia, Sophie, and Suzuki.