Besides the usual shadow box contents that haqiqat mentions, many folks now place special folded American flags in shadow boxes. The flag versions are normally triangular in shape to match the typical folded flag. In some cases the flag could be presented to a widow that lost a loved one, or it could be a ceremonial flag such as the last flag to fly on the USS Cushing or one that flew at the White House.
Some more elaborate shadow boxes combine a square medal or certificate box with a triangular flag shadow box attached to the top. These are particularly useful as a memorial to a servicemember who has passed away. The flag can be the one from their coffin at the military funeral and the certificate can hold an honorable discharge certificate and/or all of the service medals earned by the servicemember. It makes more sense to have them displayed than to hide them in a box in the closet or a bottom drawer.
You can find them on eBay or at stores in the US like Michaels. Some are beautifully constructed out of rich wood like burled walnut. They do make nice gifts if you're looking for something out of the ordinary.
Iron Noder 2017