Canyons are inherently risky. Flash floods occur without notice on sunny days. Technical skills & surrounding topography knowledge required yet does not eliminate risk.
The tourist attraction is the Flats?
The trek to Peter's Cave is a good introduction to lower Peters Canyon. Peters Canyon starts on the north side of Music Mountain. It runs about 5.5 miles(6.5 miles if you follow the zig-zags of the canyon floor) northwest to Tortilla Canyon, very close to Tortilla Flats. To check out the origins of this canyon see "Peters Trail to 2nd Saddle".
No maintained trails will be found in this area. Route finding skills are necessary. Trail data given is for the white line in the canyon, out & back. This will take you approximately 4 hours. If you follow the two red line shortcuts you can make this trip in 3 hours. Check out the map provided. This trek starts anywhere up to a mile east of Tortilla Flats. Numerous pullouts alongside SR88 are available. I recommend finding one closer to Tortilla Flats. This way you can take advantage of an excellent short cut.
From your car head straight down into the Tortilla Canyon creek bed. You can follow Tortilla Creek east, but it's nothing spectacular. If you do, finding the point to turn into Tortilla Canyon might be tricky. There's two moderately-tricky route finding points in this hike. You can skip this first one and save time too by taking a shortcut. So... from your car, head down into Tortilla Creek. Then head straight back up the south side. It's steep but you'll soon find a good trail of use. Go east on this good trail. It will take you over then down to Tortilla Canyon. This is also where Tortilla Canyon becomes distinctive.
Follow Tortilla Canyon about a half mile to Peters Canyon. The terrain isn't difficult. Only minor scrambling side to side around obstacles is necessary. Nevertheless, it slows you down considerably compared to a well traveled maintained trail. The second moderate-tricky route finding point is the turn into Peters Canyon. If you've studied you're topographical map, finding the mouth of Peters Canyon won't be difficult. It's simply the first canyon coming in on your right. It's slightly elevated so you could miss it if you aren't paying attention. Your first view into Peters Canyon is impressive and should get the adrenaline kicking in.
The easy canyoneering soon gives way to a series of huge boulders. You can make it through with some good Tarzan abilities. I know nothing about climbing ratings. So let me put it this way... you will have to lift yourself in some funky twisting, blinding, comprising situations. There's two or three problems at most. Lucky you, on the left are some easier bypasses to each problem. If you really want it easy there's a trail of use that skirts the whole problematic situation all together. I didn't figure this out on my first attempt. I'm kind of glad too, because you miss the boulders all together and they are impressive.
Just past the boulders you are greeted by a gray/white section of washboard granite. It's a pleasant visual site. Plus it's nice in the fact that you can stroll across this section quickly after the slow going so far. Near the end of this section or when the first leaning saguaro on the right appears is a landmark of sorts. You won't know it until further down the canyon. Looking back it's a face. Did I mention this canyon is filled to the brim with "Lost Dutchman" legends. Read Stewart & Carlson's "Hiker's Guide to the Superstition Wilderness" for more information.
Continue on, the going isn't difficult but it isn't quick either. The canyon bends lightly to the left then sharply to the right. You'll see the cave on your right a little ways up from the canyon floor. It's not completely visible from the canyon below. A steep but very short scramble gets you up to the cave. This is the turn around point for this description and a moderate day hike. Return the same way.
From the cave you get a great view further up the canyon. If you're more adventurous the next half mile of canyon is very impressive! If you're extremely adventurous you might try jumping the ridge over to Tortilla Canyon. This puts you in prime location to return through Tortilla Canyon's impressive section. I didn't jump the ridge myself so you're on your own. It appears possible on this side but I can't say for the flip side. By the way... If you plan on staying at the Tortilla Flats Resort while in the area, bring your own sheets!
WARNING! Hiking and outdoor related sports can be dangerous. Be responsible and prepare for the trip. Study the area you are entering and plan accordingly. Dress for the current and unexpected weather changes. Take plenty of water. Never go alone. Make an itinerary with your plan(s), route(s), destination(s) and expected return time. Give your itinerary to trusted family and/or friends.