Branches from a pine tree can be used to make a home made wreath. Popcorn can be strung for decorations.

We did paper rings when I was a kid.

Definitely paper ring garlands, popcorn garlands are a good idea too. Getting cheap gift bows at the dollar store and putting them in various places... Ditto for wrapping paper, and, if you have any paintings or large photo frames "wrapping" them like a present and hanging them back up.

We often use the previous year's garland for decorations, since garland doesn't keep well in a box.

If you do any crafts (crochet, knitting, sewing, etc) there's plenty of other options, too. I've been working on this large hanging advent calendar with granny squares.

I have been looking for ways to decorate on the affordable side for months now. My favorite so far has been these stars or snowflakes. They are absolutely gorgeous and super easy to make. I do them with silver and gold paper!

Another super easy thing I have found is making paper gingerbread garland! I decorate each 'man' as a person in my family. My daughter LOVES this craft.

You could learn how to knit or crochet and make decorations yourself. There are tons of free Christmas decoration patterns for knitting and crochet. There are also free crochet videos on YouTube that show how to crochet Christmas decorations.

I'm making chains out of the origami lucky stars that have been spray painted with metallic colors. This requires strips of lighter weight paper, string, and paint...fairly cheap. I spent about $12 dollars on enough stuff to make about 250 ft of cute little star shaped garland.