I used to buy mylar balloon on significant anniversaries for my cats -- 6 or 12 month birthdays, came to live in my home anni, etc. and when they were kittens they loved them. I have one (very blurry) shot of one of the boys dragging the balloon around the room by it's string. So for their 18 month birthday I brought them both one and they freaked!

A friend told me her cat loved balloons until it was left inside with a balloon and a ceiling fan one day. Apparently the balloon got trapped and wrapped around the fan; it was a real mess. Apparently very noisy, too, and her cat never liked balloons after that.