Ahhh... Europe. My childhood stomping ground. 
I'm not kidding about that, either. My dad was in the Air Force so I lived in Germany for 3 years followed by the Netherlands for another 3 years, and went on trips around the region to Denmark (Legoland, hehehe), Paris (and Disneyland), London, and Innsbruck, Austria. I would definitely add my vote to sticking to just a couple of cities at most. My number one choice for summertime travel would be Paris (great excuse for the wife to wear any flowy skirts she might have, as Paris in the summertime and flowy skirts are just a natural pairing, in my rarely girly opinion). The weather doesn't get too hot there (then again, I lived in Texas for four years) and it's got more museums, old churches, art galleries, cafes of all sizes, and centuries old streets than you can shake a finger at (I personally recommend checking out Le Centre Pompidou, and only partially because I think the name is awesome). In addition, it's easy access for day trips to a number of other places. If you're interested in a slightly less popular location, I took a day trip with my dad and stepmom while visiting them in Germany to Luxembourg City and had a wonderful time. It's a beautiful old city and the fortress there is amazing.
Belgium is wonderful, as well, and has some more great food (namely waffles, chocolate, and waffles dipped in chocolate), not to mention the architectural wonder that is Brussels. The Netherlands is a unique country, to be quite honest, but I loved it there when I was a kid. The people are very interesting and will ride a bike with their small-to-medium-sized dog in a basket ANYWHERE. Friendly people, though, and getting out into the countryside to check out the famed tulips and windmills is definitely worth it (I threw a fit when I almost missed my chance to tour a windmill when I was 7 because I had been pulled out of school for the final two weeks while my parents looked for a house in Florida, but the family I was staying with at that time offered to take their kids and me on a windmill tour since I had never gone and was leaving Europe in a month).
I've spent a great deal of time in Germany and always love visiting Trier, Bad Homburg, Hamburg, Colon (they have an amazing chocolatier there that functions off a river boat, not to mention the supremely beautiful and famous cathedral), and any other cities in western Germany. There is no shortage of amazing places there.
I'll see if I can get some photos from my various trips back to Europe together on flickr for you. I have at least some of the Germany photos and potentially all of the Paris photos recovered from my external hard drive, plus London and maybe a couple of other places.