One favorite experiment is the Gherkinator, essentially electrocuting a gherkin to produce sparks which glow due to the salt they are soaked in.

Glowing gherkin

I have built a couple of these, one that is very heavily interlocked and sealed inside a box, so it is safer.

This is a nice experiment to go with flame tests as a slightly more exciting example. Though playing with mains is obviously dangerous, so the first one was in a box with interlocks to make it impossible to touch anything live without turning it off.

This is great, but fogs up quite fast, so a second version was buit to be much more visible in a lecture environment. You have to press both switches at the same time which can't be done with one hand, making it much harder to accidently electrocute youself.

The body of the gherkinator is designed so that any liquids run away from the electrics into the tray and the control is designed so that you can only operate it with both hands - so you can't be touching the gherkin when it is electrofied. The forks are also removable so the tray can be cleaned.

Interlocked gherkinator
Allows you to electrocute gherkins in relative safety. The power is disconnected if you lift the lid.
Show gherkinator
A gherkinator which has much better visibility for shows, though it is no longer interlocked. To turn it on you have to press two buttons at the same time with two different hands, so you can't accidently touch the live gherkin.