My wife and her mother will be going with my daughter to Brussels (daughter will be studying abroad for a semester), and after dropping her off, will have about 5-6 days of free time before they return. They would like to visit a nice town or two in Northern France, but are open to ideas. They would prefer to see smaller towns that have some charm as opposed to large cities like Paris.
They will see Bruges, Ghent, and (after dropping my daugher off at university) Amsterdam before they have 5-6 days of free time. They will be flying back out of Brussels, else I would have suggested an open-jaw itinerary.
Other considerations: They don't care for crowds, want to keep costs low, prefer not to fly anywhere and will be taking trains, not driving.. My wife has been to Europe before, my mother-in-law is making her first trip to Europe. My wife thinks her mom would love a place like Murren, Switzerland, and that may even be an option for them with 5-6 days to play with (it was our favorite location last year). My wife is thinking about possibly going to Strasbourg, to Colmar, or to Lille (which doesn't look very charming), or in Germany, to Rothenburg and back along the Rhine (we loved the Rhine last year, stayed at Bacharach).
Aprpreciate the group mindshare!