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.