I would vote for the knives (maybe Spyderco Delica), flask or liquor. Although I really like the NRA membership idea and the lower receiver is just flat out fantastic.
I got my groomsmen 4sevens preon 2 flashlights. They come in very nice boxes that look high society and don’t need to be wrapped. The cool thing is each of my groomsmen took to carrying these pen sized flashlights on a daily basis.
If you are going to spend the money, give them something they will actually use and appreciate.
$45 a pop.
Ammo. 
Seriously though, I’d agree with the guys with the knives/lights, maybe a cool flask if they’re into that. Should be something to keep and remember. And you could also get one of the above engraved to give it a more personal touch too.
My dad still has a swiss-army style knife he got at one of his friends’ weddings…