I have had a number of ornaments broken and trees toppled over by curious dogs and cats over the years. If you can drive small nails into your walls and use fishing line or nylon thread, tie it up! Otherwise, I have friends who have chosen to just put lights and ornaments on a houseplant, like a indoor tree (shefilaris? I spelled it like it sounds). I'm tempted to decorate Boston Ferns this year as they are everywhere in my house!
The other thing I thought of was that you might use one of those cat repellent sprays mentioned in some other posts. The one my sister has used with a lot of success on her plants is Feliway. You can get it from your vet. You can look them up at www.feliway.com. I just tested the site to make sure it works and it does.
Good luck.

[This message has been edited by lhg0962 (edited December 02, 2000).]