If you are at Dhayah Fort or at the Qalat Dhayah Rest House you might have the time to look at two other things. (You probably won’t make an extra trip to Dhayah to see those sites, but if you are already there, it might be a nice add-on.)
In the plane south of Dhayah Fort there are archeological remains of several ancient tombs from the Wadi Suq period (2000-1600 BC). The sites are easy to identify, as they have been individually fenced. And while these sites might not be overly impressive to the eye, they are amongst the oldest archeological heritage in the Emirates. Several artifacts found at these sites can be viewed in the National Museum in RAK.
The tombs at the southern part of the site near the mountain slope are in a bit a better condition than the rest of the tombs.
North of Dhayah you can find some ruins of abandoned houses. Here – in my view – the surrounding mountains are probably more impressive than the ruins.