If you decide to do only one hike in the Black Hills area, then the Sunday Gulch Trail is the one that you'll want to do. It is an absolutely stunning hike and it will give you a full overview of what makes the Black Hills so beautiful.
The trail begins on the edge of Sylvan Lake (beautiful!) and will waste no time at all at impressing the unsuspecting visitor. (Note must be taken that Sylvan Lake is part of Custer State Park, and a visitor pass must be purchased at the entrance.) After a short meander through a level boulder strewed trail, the hiker approaches the downhill boulder-waterfall-oh-my-god-this-is-so-beautiful part of their journey. Hold on to the handrails, for it can get a little hairy at places. Hike down the boulders and stop every few feet to admire the mini waterfalls that are created as water from Sylvan Lake rushes over the rocks. Also along this hike are little moss covered grottoes that together with the waterfalls create a mini paradise. Very nice.
After the boulder trail, the hiker will enter a typical Black Hills forest of ponderosa pine, birch, aspen, and spruce. Wildflowers abound along the trail (even in the heat of July). Continue along the forest trail (counter-clockwise) and admire the creek that you will criss-cross on several occasions. Again, you will encounter little waterfalls along the way. Again, very nice.
Mid way through the hike, you will see a campground to your right. Do not take the fork to the campground, but continue on your left. You will notice blue diamonds with arrows marking the correct way of the trail. Whenever in doubt, always look for the blue diamonds, for they are placed throughout the trail, making it near impossible to get lost.
The trail continues uphill via switchbacks, giving the hiker opportune views of the gulch and many photo-ops. The trail levels out at a prairie meadow, again flooded with wildflowers (including mariposas!). The trail routes back towards the granite peaks, again offering yet more photo ops and more views. Very, very nice.
By now, the hiker will hear the sounds of vehicles on the roadway. Namely, the hiker will hear the roar of Harley's winding the roads because this is the Black Hills and the pilgrimage town of Sturgis is not that far away. The trail at this point rather parallels the road but never actually gets too close to it and is actually quite easy to ignore.
The trail is near its end after descending the final granite peak view point and soon you will encounter some out buildings related to Sylvan Lake Resort. Again, keep your eyes open for the blue diamonds and follow a brief paved road which empties out at the opposite end of Sylvan Lake from where you started. Take the a moment to admire the lake and decide if you want to stick around and spend the rest of the afternoon fishing or piddling around in a paddle boat. Or go for that hike again. Or go see Mt Rushmore. Or have a beer in Deadwood. Or go look for buffalo in Custer Park since you have already bought the pass. So many choices!