Wadi Shab is a must see. It is one of my favourtie souvenirs of my long stay in Oman, You will remember it as a place that is both splendid and peaceful.
Wadis (water channels whose water level can often be extremely powerful and dangerous when it rains but which varies enormously according to the season), are often heavenly places, and that's especially true of Wadi Shab.
Once you have gone through the village of Tiwi the road ends opposite the wadi. When you are at the end of the road you have to take a barge to traverse most of the Wadi. Then you begin to walk along Oman's rocky paths. You need to allow about 45 minutes walking, including crossing 2 or 3 pools of water, before you can admire the canyons and swim in the transparent water. Don't forget to take bottles of water, a swim suit and sun cream.