We are driving to Freiburg for the Christmas market and would like to go to Colmar France for a day. Is it a hassle to cross the border in a rental car or should we take the train? After Colmar we are headed towards Frankfurt. We don’t plan on spending the night in Colmar. Thanks!
There should not be any restrictions on driving to Colmar, other than any as part of your rental agreement. Check that it is OK to drive into France.
If you will already have a car then that's the way to go. It's quicker, and there should be no problem crossing the border.
Nowadays some rental companies charge a small cross border fee. Play safe, let them know in advance.