What qualifies as remaining within "Chengdu only" for the purposes of the visa?

It's a bit hard to explain the boundaries of the city. You can try by put "Chengdu" into Google. After your receive the results, you should insert it in Google maps and there you will see the boundaries of the city marked by the red line.

Roughly, you can travel to the east from Chengdu roughly 60 kilometers to the town of Wuhexiang, to the north some 20 kilometers to the town of Qingbaijiang, to the south some 25 kilometers to the town of Xinjin, to the northwest some 50 kilometers to the area that encompasses the town of Dujiangyan and two great natural areas of Mount Qingcheng and Longchi National Forest Park and to the southwest some 80 kilometers to the area of Xiling Xueshan National Park and the town of Pujiang.

You will be able to see clearly the above mentioned places on the map. The entire area you are allowed to move has 14,378.18 km2 (5,551.45 sq mi), and that is a bit larger that the US state of Maryland.

Unfortunately, Leshan is outside the boundaries, but the area of Dujiangyan with nearby Mount Qingcheng is very rewarding.