Hello, ChefUzma :)
Thanks for calling me.
Well, I thought you rather don't like to go far from Muscat...but you'd like to enjoy trekking..ummmmm.
Easy and fun trekking could be done on "Jabal Shams".
Short one would be about 1.5 hours one way, but you can make it shorter.. if you feel a bit tired, return :)
I myself did it about only a half but enjoyed a lot...wonderful (but a bit scaring sometimes) view.. as I'm afraid of the height :D
But it was in mid.June, so under the mountain was quite hot though, it was comfortable on the mountain.. however, end. December...could be cold, maybe the highest temparature would be only around 10 degrees (like 13 degrees or less/more), is it OK with you?
The place would not be dangerous (as far as you'd be walking carefully, watching your step when it comes to some narrow path), no up & down to walk, so recommended.
Oh, otherwise, maybe "Misfat Al Abreen"!!
It's on a hill but not higher like Jabal Shams, so you can enjoy the weather, too :)
I saw some signs for the trekkers (3stripes in a square (yellow, white and red, I think..)) so, as far as you follow them, you won't lose your way :)
Both are in Interior (Dhakhiliyah) region, where Nizwa is located.
If Misfat Al Abreen, from Nizwa (not Muscat) it's about 1.5 hours by car (even Saloon car is OK but 4WD is much better, especially around the parking is very steep), I suppose. And if Jabal Shams, from Nizwa it's about 2.5 hours by car (4WD is must).
If you'd like to enjoy trekking on the mountain (Jabal Shams), I suggest you to make overnight around there, or just go straight there from Muscat, without visiting Bahla, Jabrin, as it takes about 1.5 hours to reach Nizwa from Muscat already and if you intend to hire a car with driver or book a tour though local tour operator, full day tour is for about 8-9 hours and if you extend the duration, they'll charge extra, I believe...
1.5+2.5=4 hours one way, means two ways 8 hours already!! (if you go direct to the mountain, I think you can reach in 3.5 hours - 3 hours around)
So if day-tour without overnight, maybe trekking at "Misfat Al Abreen" would be easier.
Hope this will help you in getting some idea.
If there's anything, please call my name again :D
Good luck :)