I'm really lazy this year, so I took DS to the dollar store (because I didn't want him to pick something out that was too expensive) and gave him a dollar & had him pick something out. He actually made a good choice, which surprised me, but I knew DH would be happy with anything that DS himself chose.
Last year, we made a hand print & he painted a bit around it, and then we put it in a cheap frame. I was originally trying to make a hand print butterfly but I ended up having to let go of my vision & just let DS do whatever. It's still on our hutch & looks pretty cool.

Restricting the color palate to 2-3 colors can make it look nicer & less chaotic.... you can just use crayons or markers to decorate it even (and trace her hand instead of making a print), if you don't want to deal with paint...
Another idea -- glue pieces of colored tissue paper onto an ornament.
Or, she can glue the tissue paper onto wax paper, then you can make a frame for it with black paper cut out into a design, and it will be a suncatcher. Here's an ex. if you can't visualize it...
http://www.pacon.com/projects/SunCatchers.php
Another fun suncatcher idea -- put a cookie cutter on a piece of wax paper, and have her fill the cookie cutter with glue about a centimeter thick... you can also color the glue with food coloring... then sprinkle some glitter or beads into the glue & let it dry for a few days. Ex:
http://playathomemom3.blogspot.com/2011/12/christmas-glue-catchers.html