[Ramirez_the_Silvite]

hats that serve a purpose
wearing

Frog would let you swim faster, as fast as a Bomy, but better control, but you move slow on land.

Bomy Shell would make your max hit points rise while wearing

Toadstool would heal you .5 every 10 seconds, or as long as it takes a bed to heal you .5

I can't think of any others, but also some could cause a transformation.
Wearing the Frog could be like wearing a Frog Suit from Mario, that would be how it would look.