I got my father an M1 Garand and an ammo can of 30'06 ammo for it. We frequently bring the rifles out to the range to shoot. His and mine. His does have a way better stock. So, I would have to say, either a great quality firearm or ammo is always a great present.
It depends on which arm you are accessorizing and what your gift price range is. For me the ones that are budget friendly and most for your $:
Pistol Semi-auto - the biggest magazine made for it
Pistol Revolver - hks speed loader
Pistol Cap & Ball - cap strip loader
Battle Rifle - magazine couplers
Precision Rifle - shooting mat or sand bags
Shotgun - fiber optic front sight to replace bead
Cheap back pack to use as range bag. Cheaper Than Dirt has some for $40. Look at those and find something close to them. I can carry everything in it and still have my hands free to pack ammo or ......? If you do go with CTD, Do not get black, shoulder straps are way to wide and do not feel comfortable.