About half way between the 1.5 and 2.5 mile posts clockwise around the old loop, there are at least seven small Ancestral Pueblo Ruins visible from the trail west across the Rock Creek Canyon.
This pair of structures is about 0.25 miles south of 2.5 mile post. They are in the northern of two side canyons off of the Rock Creek Canyon. Below these two alcove structures there are the rubble piles of a pueblo.
Two more small sites are in the southern east pointing side canyon. The East Rock Creek Trail skirts along the edge of this side canyon, but the sites are somewhat obscured by the Pinon Pine and Juniper forest and are off to the west, not easily noticed when hiking and looking straight ahead. The site to the left looks like a good granary.
It is good strategy to spot these sites before hiking down into the canyon for closer views, so you know what you are looking for and where they are in relation to each other. (However, in 2011 the side trail down for the closer look is considered to be closed.)
The officially unnamed Rock Creek Trail (in 2011 renamed the East Rock Creek Trail) passes above these sites but they aren't visible from the trail. There are at least four sites in the first 1.5 miles of the East Rock Creek Trail, and two on the last leg returning to the trail head. With these seven, there are at least 13 small ruins to view within the 5.0 mile hike.