It is a pretty common first project to use an Arduino (or similar) to blink an LED. Which, of course, brings taunts of: you could have used a 555! You can, of course, also use any sort of oscillator, ...

There’s not much time left now. If you’re going to put something together to give the youngsters some night terrors in exchange for all that sweet candy, you better do it quick. This late to the game ...