My road trip in Oman brought me to all the 4 places. Two of them, the Wadi Park and Shisr I visited during the drive from Salalah to Muscat.
The wadi is nice, you can see the trees up close. It seemed to me sometimes guides tours are done but i could not see anyone so just wantered a bit around.
Shisr, is it worth it to go? It's a 3-4 hour drive on gravel roads (ok, i guess there is a tarmac road but that's not fun). I though it was, the place is small. An 30 minutes you see most of it for sure. Some background information is given. we did not see any other visitors when we were there.