With one day and a 12 year old, I would suggest
Hire a car
Drive to the Quiraing and walk there. The car park will be busy. But of you follow the route we did
https://www.walkhighlands.co.uk/Forum/viewtopic.php?f=9&t=72566 you will have at least a good section of the walk away from any crowds. (WH site recommends the reverse direction - one of the rare occasions I would disagree with them!)
The walk should take 3 or 4 hours (walking leisurely), and it's a wonderland for all, but especially for a 12 year old to explore - don't lose him in the labyrinth behind the Needle!)
Then drive to Carbost, the Old Inn is a great place for lunch, a classic Scottish pub right on the lochside
Then a short distance to Fairy Pools. Again there will be other people around but you can make a circular walk with quieter sections by following the WH recommended route. The pond at spot height 344m on their online map is a beautiful and totally unspoilt place. Or, if you are short of time by then, just walk to the pools and back.
Finally if you have time drive down Glen Brittle for a quick look at the sea-loch and beach, backed by the Cuillin.
If you'd like a more adventurous day, my choice of one wonderful walk would be Bruach na Frithe by the route we used -
https://www.walkhighlands.co.uk/Forum/viewtopic.php?f=9&t=72538 . Safe for a 12 year old in good weather.
Have a great time!
Tim