This is what I did... now, I'm not saying it's the best tour, but it's a start.
I visited 4 countries: Belgium, Germany, France and UK. I didn't go to Spain or Italy because I had already been there. And frankly, I really didn't feel like visiting Holland at the time.
I was also alone, so I could pretty much do what I damn well wanted. I also spent part of that summer working, in UK, so that took care of itself.
Top recommendations would be Paris, London and Berlin. I also took the chance, since I was in France and it was mid June, to go to Le Mans. There's also a theme park in France called Futuroscope which is pretty nice to go with the family. Keep in mind, except for Paris, most of France is for the French... you'll be lucky to find someone who speaks something else than French.
In Germany I visited Stuttgart and almost dehydrated myself because of the constant drooling. I also stayed with friends, so I didn't really spend much money. Berlin is an awesome city and it's quite a shock to see it, for the historical significance but also for the sheer size of it. I also went to Mannheim and Dortmund, this last one a great place to spend with family. Munich and Frankfurt were also great, and very technologically-inclined, so you can get your fix there.
One thing you might want to see in Paris, if you're there at the time, the Bastille celebration. It takes place on July 14th, and despite being a military parade during the day, at night it's usually a big celebration with fireworks (the best I've seen in my life, and I've seen a lot) around the Eiffel tower and it's a nice time to spend at the Mars camps... in general Paris is a great city.
Amsterdam is great, but more for a non-family visit. The Queen's Day on April 30th is a great celebration, and there's a few places to visit that could be family-inclined, but again, Holland in general is not a place to go with kids as a tourist.
Lastly, there's a town in Belgium called Brugge (translates to Witches), and it's the typical European old town, and they've made an effort to maintain it in the same way. It's a very pretty place.
Here's a quick pic of Brugge.
I'd recommend more in UK, but frankly, I only went to London once, stayed in Bristol and went once to Bath and to Wales... I'm sure there's many more members who can speak better of it.
Places not to go to: Eurodisney... no one speaks English and it's practically a clone of other Disney theme parks which are probably more accesible if you go to them in the US. It's also full of Americans
I might post a bit mroe about places you should visit, but off the top of my random mind, that's what comes up.