Europe: Where to go?

We are planning a European vacation for Summer 2012. It would be for 7 people (6 adults and one 1 1/2 year old). My husband and I have been to London, Paris, Rome, Naples and Pompeii but my family and best friend that are going with us have never been. This is my mom's first trip out of the United States and one of two vacations she will have been on in the last 30 years. We went to New York for 5 days last year.

I want it to be amazing for her but we can't decide where to go. It will probably be a total of 12 days. She wants to go to Rome and Paris for sure but since my husband and I have already been to those places she wants us to pick 2 more places to go. So it'll probably be 2 days in Rome, 3 days in Paris and then the rest of the time divided between the other two places we choose.

Anyone have an recommendations? Barcelona, Venice, Prague, Berlin, Santorini, Nice, Florence, and Amsterdam, have all been mentioned. Any thoughts on these places?

If it's your mom's first trip you may wish to choose cities in 4 different countries so ensure plenty of variety.

Barcelona has some fantastic architecture and you easily fill a few days there. Only word of caution I would have is to check out the situation with pickpockets. I know they have had problems and a few friends have fallen victim there.

Berlin is a great historic city and the tours are well worth while doing.

How mobile is your group, in other words, how much walking are you prepared to do? Anybody in your group have issues with stairs or walking up and down on sloping ground?

Once there how will you be traveling? Will you hire a car/van or will you just fly from location to location?

A few options:

fly into Rome, then go to Marseille, Lyon, and fly out of Paris.

fly into Paris, go to Zurich, Milano, fly out of Rome

Paris, Cannes, Genova, Rome

I almost always recommend the UK for first-time international travellers. No language barrier (sort of) and generally familiar customs and social norms.
First time travellers almost always schedule too much but I understand many people regard these trips as once-in-a-lifetime and want to see everything. That means you have a list of places you have been, but very little experience in any one of them.
If you plan to hit multiple cities, I'd recommend taking one of the general city tours on arrival to get a feel for the overall picture, and then concentrate on the areas that interest the group...and don't be afraid to split up.