In addition to the other questions - what kind of weather are you seeking? I hate cold and snow, so Switzerland would be a no for me - or anywhere else at significant altitude.
Do you want to stay in rural places or just pass through them on a train? In many rural parts, a car is the best way to get around. Also, public transport may be limited during the festive season. It’s dark by 4pm in northern Europe. It’s also likely to be wet and cool/cold.
How many nights do you have actually on the ground in Europe?
If the trip is for this year, most of the decent accommodation will have been booked months ago, so you may find that what is left blows your budget.
Depending on how long your trip is, I would suggest Lisbon in Portugal with day trips by train to Evora, Cascais and Sintra or the coast. Alternatives - Seville in Spain with day trips to Cadiz and Jerez by trainir York in England with a day trip to Durham to see the cathedral.