That's a tough one if you can get a hold of a video camera you could have all your family and friends thank him and his buddies for their service then post it on youtube or e-mail it to him, if you have a friend that's handy with that kind of
stuff you could do a slideshow type thing to music and have your messages imposed into it. Kind of like the song "I'm
Already There" if you haven't heard it you should it's a great song and they have a version that has messages to members from
their families in it. FI said pitures are always great too so maybe you could get a sign that says thank you and have friends and family and maybe even random strangers if you don't mind approaching people take pictures with the thank you sign.