Hey Don, Baltic Birch plywood is good, but I prefer Hardwoods. For ornaments I like to use Mahogany, Cherry, Walnut, Maple and Alder. Since these are personalized and you will be cutting them one at a time, I would use 1/4". Use an oil to bring out the grain and color, Then when dry, top coat with your favorite finish. I like to use semi gloss lacquer.
Good Luck and post pictures when done!