Not sure how big your tree is, but my mom used to talk about putting the tree in a playpen when we were little.
Right now our cat is not allowed to be in the living room by herself because she has taken every decoration (ornaments, poinsettas, bows, and beads) off the bottom two rows of branches. The positive note is that she has only climbed the tree once this year, which is an improvement over last year when she was climbing the tree every chance she got.