There aren't enough types it would be cool for them to be like plush and flowers with 1-2 in every flight location and maybe a new travel method on a boat that lets you bring a few more items at the cost of taking more time and allows you to fish for special items and well fish (giving the yacht upgrade a use).
The fishing would be inspired off Stardew valley and other bar style fishing and while fishing it would be a 1/100 or smth chance you could get treasure and it would be anything from junk to rare items like a gold statue or smth that can be traded into the museum
feel free to drop other suggestions that would go in this below!
Others suggestions I agree with
FROSTYKID: "Different fish as temp item, each giving something like a buff or de-buff : bleed, Poison, stun. Health boost / tick ect "
I think like a puffer fish or lionfish also maybe rock fish could apply poison or stun or some kind of new debuff.
PYTHON: "Maybe add a fishing rod item and give the PI yacht a use to catch the fish similar to hunting but get the fish as items instead of cash"
Would be an alternate to allowing the traveling by boat or the way that you can travel by boat.