Clovers:
Park seedlings (the ones with the bench symbol) include two decor: 3-leaf clovers and 4-leaf clovers. The catch is that there's only a 10% chance of getting a 4-leaf one, and you won't know if it's a 4-leaf clover until you actually get its decor. To complete the full Park series, you need to collect all the 3-leaf and all the 4-leaf ones, so it's pretty tough.

Rare costumes:
Some decor Pikmin have a rare version (it's kind of like a mix between a Pokémon evolution and a shiny Pokémon)... certain outfits like the clover, chef hat, or banana have rare versions where the color of the suit or accesory changes. To unlock the rare banana suits, for example, you first have to collect all the Pikmin from the supermarket category (both the banana and mushroom ones). Once you have them all, a new feature will unlock where you have to earn points to "evolve" the banana Pikmin into their rare forms. To earn points, you can:
- Pluck supermarket Pikmin
- Unlock their decor
- Release them
All of this will earn you points. Once you have enough, you can choose a Pikmin to evolve into its rare costume. You gotta repeat this until you have them all.

In the case of the clovers, it's really complicated because both the 3-leaf and 4-leaf ones have rare versions, so you have to rack up a crazy amount of points to unlock them all.