I don't know about anytning that would sustain activity like a treadmill, but my little one will chase a toy as many times as I will throw it. One he (and I) love is a little squeaky pig made by kong. It bounces in every direction (ona hard surface) with no way of knowing where it will go. Huey has a blast trying to catch it as it goes everywhere!
About.com: http://www.kongcompany.com./
We also have the hippo, but the pig bounces more erratically. He loves both. They are perfectly sized to fit in his mouth-he is 7.5 lbs- and they get slippery when slobbery so he can be running and it pop out of his mouth and start bouncing around again. He never gets tired of them.
Sorry for the long reply, and welcome to YT!