We all have our favorites. There are also riding opportunities at many local reservoirs but most of them are not maintained as riding areas and do not have designated horse trainer parking areas. Get to know riding trails by joining organizations that sponsor rides at sites you do not know.
Codorus is a great place to ride. The trails are well-maintained and wide with picturesque vistas of the lake. A 40-trailer parking lot provides plenty of space for a day visitor. The volunteer organization Pennsylvania Rangers helps maintain the trails, promotes riding and sponsors many equine events in the park.
I like to go there at least twice a year in the spring and fall. That is when Dogwoods blossom and the fall leaves are at their peak. In the dead of winter a cold wind can blow over the open battlefield and the deer flies can be very bad there in the heat of summer. However, there is a lot of history there to be seen from horseback. The volunteer organization Gettysburg Equestrian Historical Society helps maintain the trails, promotes riding and sponsors many equine events in the park. A private concern Artillery Ridge Campground has great facilities for a person visiting with their horse at reasonable prices and the park also has a horse trailer parking area in McMillan Woods off of Confederate Ave. For a virtual tour of Gettysburg trails see my page here.
I have only been there a couple times and cannot say too much about it except it was a nice place to ride.
The McKeldin Area offers woodland trails. The parking is very good but there is a modest admission fee. The trails closer to the park headquarters are well-maintained but during prime time full of foot traffic. Farther out pedestrians are rare but the trails less well marked and maintained.
Michaux State Forest offers exclusively woodland trails. It is wild and beautiful in season, but I can honestly say that I have never gone there without becoming very seriously (will I ever see civilization or another human being again?) lost. There is a large parking area but the access road from the east is narrow and uphill. I have been told the riding trials are marked but maybe I was so lost that I was never was on any of them. Avoid at all costs the Appalachian Trail, where horseback riding is not permitted and punishable by fines. There are nice maps but they are useless with no physical features visible in the woods on which to orient. I do not advise going to Michaux unless accompanied by another rider who knows the trails well.
Morgan Run offers a variety of either woodland or open trails. Long straight, wide lanes through a failed reforestation project allows for some nice long canters. The parking is very adequate. The volunteer organization Carroll County Equestrian Council helps maintain these trails, as well as other riding sites within the county. During the summer watch out for ticks, our horses pick up more at Morgan Run than any other place we ride.
Sweet Air is mostly woodland but there are some open stretches appropriate for a canter. The park was posted carved wooded "You Are Here" trail maps at multiple points and also has build many erosion control measures along the trails. These thoughtful measures might serve as models for many other less well managed sites.
The Union Mills Trails are most entirely woodland. They can make for a very pleasant ride but a first timer might get lost. The parking areas are relatively small and located on either end of the park. The northern one is off of Kowomu Trail Road and the southern and smaller one is off of John Owings Road.