Update: This cruise line is no longer in business.
If you want to really dig into the history and culture of a destination, Voyages to Antiquity is your cruise line. As you may have guessed from its name, the line started with itineraries in Greece, Italy, and Turkey but has since branched out into Asia and Africa. Its single ship, Aegean Odyssey, carries a cozy 350 passengers and cruises two-week itineraries on average. Entertainment consists of small musical acts, which aren't nearly as popular as the many onboard lectures. There's a single dining room, plus an open-air buffet restaurant, both serving essentially the same offerings. Overall, the mood is low-key and casual.