Use sunset as your turnaround alarm.
The trail here is the river. You walk the paved Riverside Walk for the first mile, then step into the Virgin and keep going upstream, and the canyon closes in the further you push. No permit is needed as long as you turn around at Big Spring, which is the real distinction from the top-down route. Park biologists have found toxic cyanobacteria in this water, so treat it as something to walk in and not to drink or filter.
Wade UP the Virgin River between 1,000 ft walls. Go as far as you like - even a mile is magic. Rent canyoneering boots in Springdale.
Heat note: desert country - in summer start at dawn and carry more water than feels reasonable.
2 liters water, trail shoes, sun layer, snacks, small first-aid kit.
Make it a weekend: South Campground (Zion) (Camping, 6 mi) · Watchman Campground (Camping, 6 mi) · Lava Point Campground (Camping, 8 mi) · Kolob Reservoir (Fishing, 12 mi)
More hikes nearby: Riverside Walk (Hike, 0 mi) · Angels Landing (Hike, 1 mi) · Emerald Pools (Hike, 2 mi) · Observation Point (East Mesa) (Hike, 3 mi)