The Grotto is Nice. The Grotto is a wonderful, drippy overhanging grotto type alcove, back away from the road at the Grotto shuttle stop. The Grotto itself is a very nice place to hang out on a hot Zion afternoon. The canyon leading to the Grotto - not so much.
Grotto Canyon descends between the Great White Throne and Red Arch Mountain ending at The Grotto. It is not a very appealing canyon. Since it gets very little water flow, it is more like a fault-following gully than a canyon. I did it, so you don't have to.
The road used to end at The Grotto, and is the site of the first ranger cabin and the first park campground. If you are camping in South Campground and wonder where the North Campground might be, think The Grotto.