Guitar and all fretted instruments for 40+ years and keyboard noodling.
Always have at least 3 or 4 string instruments in my edit suite for unwinding and decompressing.
Love to play with a couple of my sound buddies Dino DiMuro and Alan Rankin.
It's funny you ask because years ago I noticed that most of the people I work with are frustrated musicians.
I started as a trombonist and moved through all of the low brass instruments before picking up a bass guitar and eventually a sound effects library! If you are noticing a pattern, I love the timbre of instruments in the bass clef!!
As to how it relates to Sound Design, it is all about what that experience does to your ability to perceive (and control) ...