Fiddle. It's so versatile and blends well. I've long given up thoughts of playing one.

The instrument I've most toyed with trying to play in more recent times is the button accordion, B/C or possibly C#/D (I did once upon a time dabble with D/G melodeon). I'd probably want to start off with something like the Irish Boube and wouldn't want anything hard work for Irish reels so cost is one stopping point for me at the moment. Also there is the recurring question as to whether these days I'd remain dedicated enough to learning.