On the way back to Muscat from Sur and Ras al Jinz turtle watching, within an hour or less are two wadis that are recommended for hiking. Wadi Tiwi first, then Wadi Shab. I went only to Wadi Shab and it was GLORIOUS.
Hike is about 3 hours roundtrip including lots of time for swimming and exploring the partially-underwater cavern. There are parts of the hike where you have to swim so don’t bring any valuables. If you bring a camera bring a waterproof bag for that. There is a place you can jump into the water early in the hike from a height of 5 meters, a little thrill and then you are nicely soaked for the hot part of the walk. Then you keep hiking up to the partially submerged cavern with a waterfall (the entrance is very very narrow, almost requires swimming underwater to get in). You can climb up the rock wall with the rope and reach another cavern with lots of niches to explore, and then on the way down jump off the waterfall. It’s just amazing in there.
I highly recommend taking Hussein as your guide for the walk, he was really friendly and knowledgable and his little agency is trying to keep the area clean without much help from the government. He doesn’t intrude on the dynamic if you want quiet hiking or whatever but adds the most interesting parts of the hike. He points out the jumping places, where you have to swim from the path, the interesting parts of the cavern, which you would definitely miss without a guide. You can usually find him with the boat people under the highway at the car park. There are other guides there too if he is not available.
Don’t wear flip flops since the hike has some steep rocky places but wear shoes that you can walk into the water with, and shorts and shirt that you can swim in (not a problem for women walking in shorts here, it is very secluded except for the tourists). We just walked right into the water in our clothes for the sections of the path that are underwater.
To get there, the exit is marked clearly from the highway, but then when you go down the offramp, it doesn’t offer Wadi Shab as an option—the key is to take the small road (all the way to the right, going back the direction you came) marked to Tiwi, then it starts showing signs to Wadi Shab again, it’s just a couple minutes til you park underneath the highway and take the little boat (200 baisa) across the lagoon and start the hike.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.