I have a single 35lb (16kg) kettlebell. Usually I spend 10-15 minutes doing kettlebell snatches and swings (switching between them as I tire, trying my best to keep moving continuously). I throw in a few other assorted exercises too, but those two exercises are the core of what I do and seem to have helped significantly with my lower back in particular.

There are a lot more exercises you can do if you have two kettlebells. I'm probably going to get a second sometime in the next few months; right now I'm still going pretty strong with just one though. Also, the 35lb kettlebell I had was probably too heavy for me for the first week or two I had it. It is the advised starting weight for men but it was a rather difficult ramp-up for me. I don't really have trouble with it anymore though.