We did a Beatles cruise once (by accident, we didn't know when we booked). They had a great tribute band and a lecturer who did a nationally syndicated radio beatles show. It was fantastic; especially as there were only 144 pax onboard and the girls (ladies, women) all sat in front row and screamed.
What would interest me are some specialised food cruises; e.g., I would love to go on a cruise that was devoted to foie gras. I have been on a chocolate cruise before (with Godiva chocolatier) but I would prefer foie gras. Also, a whisky tasting cruise would be great with the ability to work through 30 or 40 different scotches over a week.