I love to decorate with trees. Here are some ideas for themes.
Snowmen There are all types of snowmen find your favorite and use snowmen garland and other snowmen ornaments to decorate with
gingerbread. This is the theme I use in my kitchen. I have small gingerbread men, cookies, and other ornaments.
Red and white candy or peppermint. For this tree I use red garland red ornaments, peppermint ornaments,
Here are some others,
Color theme. This can be black and white, light and dark green, all silver
How about a hobby theme. sewing tree with small thimbles, thread etc.
a western theme- boots, cactus
Check out Hobby Lobby for a variety of themes.