December in many rural or islands of Scotland can be dismal. Lots of places closed for the season, hotels 95% empty, west coast can be minging wet....
Edinburgh would be good, or come up a little too Stirling. There's the Trossachs if you get a nice day to see lochs or climb a mountain, Stirling has the castle, the Wallace Monument, some museums, nice restaurants and cafes. Edinburgh by train for a day out. A more adventurous place....
Edinburgh is ace in winter. From great shops and cafes, to nice botanic gardens, Dean Village, modern art museums, portrait gallery and more (all free...), clamber up Authurs Seat etc. You won't go wrong in 'burgh.
Both are very accessible from south of the border, and save an extra few hours schlep up to Cairngorms or out to Islands for what is only a weekend...