Cove Valley service plaza...
Here's what that looks like as of fall 2013–standing near the re-entry ramp and looking eastward, back toward the plaza. The two parallel lines of saplings and brush (going away from the camera) are where the fuel pump islands would have been located, and the dense square of trees and brush in the mid ground is the location of the former plaza building–which is surrounded by pavement on all sides. It's hard to see from this shot, but there's a large expanse of pavement (the main parking area) further behind.

The photo below is from the opposite angle, standing at the exit ramp from the turnpike to the plaza. The Jersey barriers, turned 90° to the direction of travel, barricade what would have been the car entrance to the plaza. The truck entrance was to the right of that overgrown patch of trees, and there would have been a " <–CARS | TRUCKS –>" sign assembly right there. Around 2005, an approximately 4 ft. high and 10 ft. wide ridge of dirt running the entire length of the plaza area appeared over what used to be the curb and sidewalk separating the westbound lanes from the service plaza parking lot. As you can see, it has subsumed most of the right-hand through lane.

In addition to Cove Valley, the PA Turnpike has several other decommissioned and demolished service areas. The most similar to Cove Valley is Path Valley, located on the eastbound side just east of the Tuscarora Tunnel (
http://bit.ly/1d8WIBJ). Path Valley was open until at least the early '80s, but business suffered greatly as motorists were drawn to the newer and larger Sideling Hill plaza (which served both directions whereas its predecessor served only westbound traffic). I found an article (
http://bit.ly/1dkKlnC) in the Pittsburgh Post-Gazette where Exxon and Howard Johnson's (who still had the concession contracts at the time) actually petitioned the PTC to close that plaza because it was a money-loser.
There were also plazas like this one (
http://bit.ly/1aoitwK) near the US 222 interchange (Reading) where the plaza building was not demolished but instead sold to a private company and reused for other purposes. The former Denver Service Plaza is now Denver Cold Storage. I believe there are a few others that are still standing but used for another purpose–one just west of the US 15 interchange (
http://bit.ly/1a7Tm0z) and I think another out west near Pittsburgh. There may be others.