At some point, before Halloween last year, we ended up with a plastic cauldron. I thought it would be fun to have fog bubbling out of it, but rather than deal with dry ice, I decided to look for an alternative.
And sure enough, I found the FITNATE Mist Maker on Amazon for only $16 (the price has since dropped closer to $11). The device is rather simple. It generates a small bit of mist/fog using ultrasound to atomize water into a light vapor. It requires a very small amount of electricity and needs nothing else more than water. The built-in LEDs help to create a more otherworldly effect.
I’ve tested it in a sink, a couple of times and yes, I also used in a cauldron around Halloween. It worked great for our purposes. It’s a great price and worked perfectly. Note that you’ll get a good amount of fog for something like a cauldron but don’t expect to get enough for anything much larger, like a small stage.