Thank you is completely inadequate: You are the silver platter upon which our lives and freedom have been served to us.
This day is not about those who now serve or have served. For those good people there's a holiday: Veteran's Day, and hopefully a 'thank you' from someone for their service every day of the year.
Set a place for them at your table this day, that your children might ask, "Whose place is that?" and you answer "For all those who gave their lives throughout America's history, that we might sit here free to say grace to G-d, or not to say it. To be free to live our lives and not fear a conqueror or an autocrat. To express our ideas freely..." and anything else you wish to add.
This is what one American did to say "Thank you" to those who gave their lives for us. Her work will probably never be appreciated more than by the loved ones of that legion of young men and women who receive our gratitude and pride in their sacrifice on this day, and then fade back into the shadows and the chronicles of their nation.