The brick building is an old water pumping station. There is a large concrete storage tank at the top of the hill above the building and slightly off the trail (it's visible from the trail if you know where to look). I think it was for the old hike-in camp sites and bathroom buildings that were at the park in the 80s. All the concrete octagons scattered throughout the south side of the park were fire pits for these campsites. There are also two concrete foundation ruins among the south loop for the old bathrooms; they a little harder to find though.
Another Illiniwek ruins POI: At the top of the overlook on the north side stands the remains of a tow rope support tower from when that area was used as a public ski area.
⚒ Trails don't build themselves. ⚒