After reading this question (and seeing the part about how items are not effected by stale moves), I wondered what moves and attacks count as item. Some of them are obvious, like Richter's Holy Fire or Peach's Turnip, but what are the extent of what the game considers as items?
Here is a list of all characters with moves that will generate an item that other players can grab. I tested all of them in training mode. Unless noted otherwise, none of these will stale:
- Mecha-Koopa - Bowser Jr. Down Special
- Banana - Diddy Kong Down Special
Crown - King K. Rool Side Special
- This move will stale when used as a special boomerang-style. While it's possible to pick up and throw if you don't get it back, unfortunately K. Rool can't pick up the crown himself to use as a projectile, he just places it back on his head.
- Bomb - Link/Toon Link/Young Link Down Special
- Metal Blade - Mega Man Neutral Special
- Bonus Fruit - Pacman Neutral Special
- Turnip - Peach/Daisy Down Special
- Gyro - Rob Down Special
- Tome/Levin Sword - Robin Specials/Aerials
- Depending on OP's definition of "special move counting as an item"
- Holy Fire/Aura - Simon/Ricther Down Special
- Grenade - Snake Neutral Special
- Wario Bike - Wario Neutral Special
- Throwing the bike will never stale, but riding it into people will.
That's all the items that I was able to find, but if I've missed any you think might work, let me know and I can test them out!