Every few months, VisitEurope.com, the European Tourism Commission's website, publishes a roundup of country offerings for specific interests. The most recent one is about destinations that offer a wealth of family activities.
The ideas range from accommodations (in Switzerland, the whole family can stay on a working farm where kids can sleep in a hayloft and help the farmer milk the cows) to activities (in Barcelona, kids can do arts and crafts and watch performances at El Poble Espanol, an open-air-museum village that aims to capture the spirit of Spain in one place).
Traveling with kids in Europe is a balancing act for parents. One of my favorite stories about this is that when my boyfriend was young, his parents worked out a deal for traveling around Europe whereby they would visit museums one day and then playgrounds the next. It kept everyone happy and looking forward to the next stop.