Colmar town centre is very nice. Even if you are only there for an overnight you can have a very nice stroll round, look at the churches and cathedral, see the 'river' (really a stream). Plenty of nice places to eat but I can recommend this place
http://www.restaurant-pfeffel.fr/accueil/restaurant-colmar-alsace-pfeffel-accueil-en.html
We have had several lovely evening meals here. They do an a la carte and several fixed price menues. Thier local wines are also very nice.
I'm sure Alsacienne will be on this thread soon to extoll the virtues of Colmar.
I should add that when we did the Italy run we did our frst overnight in Colmar and our 2nd in Cremona where we used the Ibis which was very close to the town centre.
On the way back we did our first vernight in Colmar and our 2nd in Dunkirk getting the ferry back the following day.
We crossed Switzerland and instead of taking the Gothard tunnel we went over the Gothard Pass. Going down we did it in glorious sunshine. Coming back we did it in a thunderstorm. Both journeys were spectacular!
You will need to buy a Swiss tax disc (Vignette) at the border as you enter Switzerland. This is valid for the year. Sometimes you can buy these on the ferry. We did the Dover/ Dunkirk route. Longer crossing (2 hours) but very cheap (in fact we upgraded to 1st class and it was sill cheaper than the other crossings). It gives you a good run through Belgium, Luxembourge, and into France to get to Colmar.