Ultralight spin fishing is a natural (IMHO) progression for the fly fisherman.  The right UL outfit can allow one to cast “flures”, which is shorthand for “flies”.  Lefty Kreh, Dave Whitlock, Joe Robinson and many more “famous” fly fishermen all have played a hand in UL fishing here in the US.  UL is perfect for fishing deeper runs, or for casting under overhangs, or just to change things up a bit.  My favorite setup right now is a 6′ TFO UL rod with Tennessee grip and slip rings, matched to a vintage Shakespeare 2200.  There are many great lines out there, and Trilene XL 2# is my go to – it’s inexpensive, well made and widely available.