There's some great tours - you can do a Danube river tour from Budapest to Hamburg via the Rhine (sp?)

I'm a huge fan of Italy, especially Florence and Venice. I hear Sicily is great and I'd love to go to Puglia (google grand designs abroad puglia )

Would they be interested in going to Gallipoli? You could take a month to do Turkey, it's fabulous.
Eurail passes are worth looking into. Do they want to self-drive and hire a campervan or prefer tours and trains?

I find with planning a tour, to find what interests you and build it around that. You can only take so many castles, cathedrals and paintings When I first went to Italy, I based my travels on seeing as many of Michelangelo's creations as possible. It was a great pretext that took me to places I wouldn't have thought of (small, winding lanes and squares in Rome with hideaway churches holding the most exquisite sculptures).

Sounds like you'll enjoy helping them make plans