Best US Fly Fishing Spot to Gift a Flight In March?

Looking to buy my dad, an advanced fly fisherman on the East Coast, a trip somewhere in the US to go fly fishing. Noting he is in as good physical condition as you can be at 65!

He fished in Colorado this fall but hasn’t done much outside the Northeast to my knowledge. I don’t fly fish but am planning to join him, so bonus points for locations with good road or trail running! Thanks in advance!