Things to add? Jingle bells tied on with ribbons? Decorate your knitted tree with anything tacky you can find to add on to it? If you could find two sweaters (if they are cheap enough) in different colors you could maybe mix up the pieces from them and have different colored sleeves or parts of sleeves? This sounds like to much fun! While at the thrift store you might look for some tacky tree ornaments you could tack on (pun intended!) A felt or fleece snowman? What's Christmas w/o a snowman?
I should never overestimate my basic arithmetic skills.