What household appliances are most likely to set off a Geiger counter?

Much to my surprise, I encountered a refugee Geiger counter (from circa 1965) when I returned home from work. Naturally, I fired it up and started wandering around with it.
To my mixed disappointment and relief, I couldn’t get more than a couple of desultory clicks out of the device, seemingly at random.
Can you think of anything else that I might have overlooked?

The more pragmatic side of my geekiness wants to see if it’s functioning properly, hence the effort. I know that I could sweep the house a few times and see if some areas came up positive more than others, but I don’t feel like it right now.