I've dabbled with multiple instruments in my life
off topic celebration of playing by ear
but I've been acoustically crippled with a bad ear. About 2 years ago, I started working on playing by ear and, holy shit, it totally changed the game! Before, I had to laboriously read music and look up fingerings. Now, I know what it's supposed to sound like and I keep pushing buttons (clarinet) until I get it right because I know what it's supposed to sound like, and now I know if I want to flat that position it's this button, and to sharp that note, it's this button, etc. Quite painless and not laborious--even fun!
It seems crazy to me someone would learn all the fundamentals of music and not at least be able to play rudimentarily another instrument. I mean, why not? You already understand music, you just have to figure out the instrument, which is a pretty small investment relative to musicianship, and opens up a lot more possibilities.
What have you learned from your different instruments?