1 Tbs cinnamon, ground
This is as free form as you want to make it. Have whole spices instead of ground ones, use them. Conversely, only have ground spices - use those instead. Add the smells that you like. Don't use those spices. Only have dried fruit skins or zest - use that instead.
Just WATCH the water level. I have been using a 2 quart sauce pan and have had to refill the water about every 2-3 hrs. I'm so ADHD that I have to set a time to remember to check the pan!
Next time I make this, I'm going to put the tree sprigs on a paper plate and cover it with another one and use a hammer to beat the pieces to release the oil in them. I think I'll have more of a "Christmas tree" smell if I do.