Hi A.Beer and welcome to the forum.
Sorry no-one has replied before but I'll do my best and hopefully you haven't booked anything yet or are flexible!
I would suggest either The Nevis Range
or Cairngorm. Both are an easy train journey, you'll probably have to change in Glasgow
and maybe again in Perth for the normal trains but you may be able to get the sleeper straight through to either Aviemore (for Cairngorm) or Fort William for Nevis.
I prefer Cairngorm personally (but maybe because I've skied it more). Lots of accommodation in Aviemore or Coylumbridge, good cafes, bars, restaurants and a bus service from the town to the slopes (31) which runs hourly I think. Also a bunch of taxis who will take you up to the resort. There is quite a bit of snow up here at the moment (most of it on my driveway it seemed this morning) but there are lots of other things to do in Aviemore (low level walks, biking, spas etc) if conditions are not idea.
You can also access the Nevis Range from Fort William on the bus but not sure of the timetables, there'll be more taxis than Aviemore as well. Lots of accommodation and bars/restaurants as well.
For me Fort William feels a bit bigger, dare I say less friendly? The skiing is good, I haven't done so much walking around there (lots of white water kayaking but I'm not sure that's what you're looking for!) but there will be lots of great walks around.
For the northern lights, I've never seen them but a mate of mine has taken some great pictures this year from as low as Loch Lomond
so you may be lucky. Fort Bill is higher up the country than Aviemore so that may be a variable to add into the mix.
Whatever you choose to do be flexible in your plans. Scottish skiing can, and often is brilliant. Not the pristine corduroy of the Alps but really good fun and with some of the piste signage, a bit of an adventure! However it can also be dreadful. I can't believe anyone loves skiing so much that they enjoy a day in the pouring rain and howling wind up here!
I haven't mentioned the other 3 ski resorts up here (Glencoe
, The Lecht
, and Glenshee
) which all have their charms (Glencoe is my favourite of all) but are tricky to get to on public transport.
If you need any more info feel free to PM me.