Cities mostly, Rome, Venice, Berlin, Amsterdam, Paris, Stockholm, Copenhagen, all for short breaks of 2,3,4 days. Plenty of things to do and see, and in a city, there are always lots of "single" people around, being residents, workers, etc., so you don't stand out and shops/cafes etc don't even notice you're on your own.
Most difficult "singles" holidays are attractions, theme parks, beach resorts, etc., where you're literally surrounded by couples, groups, families, etc., and stand out as being on your own and where you are more likely to get hit on by local sleezy staff etc.
As for a cruise, being single on a cruise is my idea of hell as there's going to be an assumption that you're "on the pull" and being allocated a seat in a dining room, at a particular time, with other random singles is simply not going to happen. I'd eat all my meals in the buffet instead so I could go at a time of my choice and sit on my own.
I suppose it's all down to whether you want to meet and do things with other people. If you're looking for a partner, or want to do things with other people in small groups, then, you're looking for a very different experience to someone who's happy doing things on their own.