Definitely not summer!
Being someone from a tropical archipelago, I enjoyed a desert trip to Sharqiyah, where you could go sand buggying and camel riding etc.
Oman also has lovely wadi oases that usually come with incredible geology, so good hiking (just make sure you manage the heat stress well).
My colleague there recommends Jabal Akhdar for a more highland climate to escape the heat, I haven’t been.
Oman also has historical places in the interior – like Nizwa and Bahla.
All of these are reachable from Muscat as a base (but you will need a car).
If you go all the way south to the Salalah area (probably need to fly from Muscat) it’s said to be more humid. I haven’t done this yet. But I know there are turtle beaches there.