Are you planning to hire a car or 4x4 and explore yourselves or use taxis or use tour operators? as it all depends.
The following link is a start to give you some ideas: http://www.goway.com/africa/oman/index.html also http://www.omanet.om/english/home.asp
The weather should still be pleasant enough and not very hot yet. I suggest you spend may be around 3 days in Muscat for sightseeing ie Muttrah Souk, diving in Oman Dive Center, some relaxing / swimming / snorkelling on Muscat beachs, Grand Mosque, there couple of Museums too. You can also go to Nakhal Fort, Nizwa area for Fort/Souk and old village Misfat Al Abreen, Al Hoota cave, Jabal Al Akhdar, Jabal Shams, etc. Sawadi beach for camel/horse riding and they can also organise diving trip to Damiyant island. For a dessert safari, you can go to wahiba sand.
There is also Sur for turtles and a Wadi Tiwi / Wadi Shaab just before Sur, which are worth the visit.
I dont thinkthat you have enough time to combine Salalah as well, which is about 1000km away from Muscat. You probably need another 3-4 days to cover it.
Hope that help you and if you need more or specific questions, you can contact me directly.
Good luck and enjoy.