See Bernheim's website for updates on closures and hours.
Follow the mowed path as it exits off the Two Ponds Loop. The Prairie Trail can also be made into a loop, by following Arboretum Way back to the main path, or you can make this an out-and-back. Make sure you head all the way out to the Big Prairie Stand and get a above grass-eye view of the prairie. Then you can continue over to the Earth Clock. It's an interesting sculpture. Getting a map from the visitor center or online is a good idea, as the trails out on the prairie aren't well signed.