Thanks for all the compliments and ideas! After consideration I think I'm going in the direction of decoration, in lieu of more stuff. You could almost never stop thinking of stuff to add in there, so I'm going to keep it simple. Maybe a small cleaning kit, and probably a dessicant pouch. Maybe a couple of receipts as someone suggested.
In terms of decoration, I talked to this guy and he's going to paint the underside of the lid:
http://tinyurl.com/mz9d576
Check out his art. I've known him for a long time. He's an awesome artist, and super creative. It won't happen for about 6 months, but I'm excited about it. I guess I've got quite a bit of time to think of something specific to tell him to put in there, but I think I'm just going to let him do whatever he wants. It's art, and he's going to be a famous artist someday, so what the heck? It's a nice blank canvas, and will make the case even more special having a real work of art on the inside. And maybe he'll even do something on the top of the magazine box lid to bring it all together.
Maybe I'll just set the mood by saying "If bad guys were outside and you had to throw open the lid and start assembling AR-15's, what would you want to be looking at?" Something glorious for certain. Naked women probably.
It'll be cool. I trust him to do something awesome.
He said he already has an idea of what he wants to do, probably something art nouveau. And I didn't know what that was so I googled it, and I agree that would be very cool and kind of old school looking.
http://tinyurl.com/n2eomp2