Coins may have their place, but they've been well abused by "cheapest bidder" product and produced in numbers so great that a commander (or RSM, etc.) hands one to every driver and clerk or staff officer they walk past, just because they're trying to empty their pocket of that visit's allocation of coinage. But a coin won't satisfy the vocal crew wishing for a bit of chest bling.
A lapel pin could work, with the right official backing. It would be as valid as the recently produced pins that get handed out on retirement from the Reg F with all the other Depart with Dignity items.
It could be as simple as a quality design with the tri-force emblem and a banner beneath saying "I Served". Authorize the design, produce it centrally, and let those who want one to wear purchase it. Secondly, authorize a design pattern and let regiments and corps produce matching designs with their own badge emblems. But, and this would be the important aspect, copyright the basic pattern as an official display of having served, and make the wearing of one fall under the same regulations as wearing a medal one hasn't earned (i.e., wearing it to imply that you did earn it). Make the symbol official and recognized by the public ... and then defend against it's use by others like the Legion does the poppy symbol.