Ben has made some excellent suggestions.
Canyonlands Needles is a fabulous destination for teenagers. The fun & spectacle per mile hiked is unmatched. There are adventurous little routes through nooks and slots, up and down some ladders, but all well marked and easy enough for a confident 8-year-old. The rock formations look like they are sculpted by Gaudi. The season for good hiking is really only mid March – mid May, for reasonable temperatures and water, in this period it gets heavily booked, for this year your choices are already limited –
https://canypermits.nps.gov/
Coyote Gulch is also a great choice for adventurous teenagers, and does not require permits.
Buckskin Gulch is also one of my favorite places in the world, but there are sections of thigh-deep wading in muddy water, a somewhat tricky boulder pile to scramble through toward the Eastern end before you reach the Paria, and no potable water until close to the Paria. And there’s the psychological element of a continuous 12-mile-long slot canyon with no way out, which can make people uncomfortable. The hike from Wire Pass all the way down Buckskin to the confluence and back out up the Paria to the White House trailhead is a great 2-day 1-night hike, but I would not take an 8-year-old all the way through, even a very adventurous one. I’d stick to an out-and-back dayhike from Wire Pass where the kids can go as far as they feel comfortable. This can be hiked at warmer times of year because the slots gives some shade, but check the weather forecast carefully – don’t go in even on a dayhike if there’s even a remote possibility of thunderstorms in the catchment.
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