I got mine personalized mason jars with a 6 pack and also picked up their tux. The mason jars are pretty affordable and easy to find online via Google.
I think the best gift you can give is engraved flasks, as long as they are drinkers. The one problem is that they might have gotten this gift before in a wedding, as I did.The thing was that getting the second one was just as special as getting the first to me.