Many great suggestions and ideas...I'll add a few of mine from my travels in Europe.
Of course, all the great cities: London, Edinburgh, Paris, Amsterdam, Brussels, Copenhagen, Rome, Venice, Florence, etc.
England: Bath (and the roman ruins), Cotswold area, Kew Gardens outside London
France: Nice and Monaco, Provence
Holland: Keukenhof Gardens,Lisse (world's largest flower garden, especially tulips)
Germany: Heidelberg (castle ruins), Rothenburg ob der Tauber (medieval walled town), Baden-Baden, Black Forest, Berchtesgaden, Garmisch and the Zugspitze, Nuremberg, Munich
Switzerland: Lucerne, Interlaken and the Jungfrau region (Grindelwald, Lauterbrunnen, Wengen, Murren, Trummelbach Falls)
Austria: Salzberg, Innsbruck
Italy: Tuscany, Pisa, Sorrento and the Amalfi coast and Capri, Lake Como area
No matter where you go, enjoy yourselves and have a great time. Be safe and watch out for pick pockets, etc. Please let us know how it all went and post some of your photos when you get back.