We hang large individual jingle bells on the bottom branches of our tree. It actually works pretty well (presuming the sound of jingle bells doesn't drive you crazy.)
The cats play with the bells and almost never climb up into the tree itself (but I make no promises about stretching a paw w-a-a-a-y out from the arm of the couch to take out an ornament here or there.) And if they do climb, the bells narc them out, LOL.