Use sunset as your turnaround alarm.
This hike requires a permit or fee. Check current rules before you go: recreation.gov or the managing agency.
Most of Zion's big hikes just ask for legs, but this one asks for a permit and a tolerance for exposure, and the two are separate problems. You climb from the Grotto up to Scout Lookout on your own, roughly 2.25 miles and 1,000 feet of it, and only the final half mile of chained fin needs the lottery permit. Rangers can ask to see that permit anywhere on the trail, so download it before you lose signal. If the chains rattle you, Scout Lookout is a fine place to stop.
Chains section along a knife-edge fin to Zion's most famous perch. Permit lottery required.
Heat note: desert country - in summer start at dawn and carry more water than feels reasonable.
3+ liters water, broken-in boots, layers for weather swings, headlamp, real food (jerky, trail mix), trekking poles, first-aid kit, and an early start.
Make it a weekend: South Campground (Zion) (Camping, 5 mi) · Watchman Campground (Camping, 5 mi) · Lava Point Campground (Camping, 9 mi) · Kolob Reservoir (Fishing, 13 mi)
More hikes nearby: The Narrows (Bottom-Up) (Hike, 1 mi) · Riverside Walk (Hike, 1 mi) · Emerald Pools (Hike, 1 mi) · Observation Point (East Mesa) (Hike, 3 mi)