So much will depend on your interests. If you are wine lovers and find yourselves attracted to Tuscan design, take a Med cruise. If you are fascinated by what once was hidden behind the Iron Curtain, take a Baltic cruise. And there are still many choices to make.
Why not see a travel agent who knows cruises and Europe? After some chat to get to know you and your interests, and those of your group, they should be able to help you narrow those choices down to what will be the best ideas for you.