We recently bought a large tub of various shaped silver cookie cutters for baking or play dough and our daughter was playing with them while we were decorating our tree. Inspiration struck:
Since we were low on decorations and the hollow cookie cutters are so easy for a two year old to place on the tree...instant decorations. She had so much fun putting them on and taking them off without any hooks or small strings to place over the branches- just the whole cookie cutter right over the branch. They're lightweight and they're shiny. You can use all different shapes like we did or stick to holiday related shapes. Even if you don't go all out, let your toddler add a few of their own "special ornaments" for fun.
(Image by Flicker member stephskardal licensed under Creative Commons)