I've done this before as an instructor as a tech camp. One great activity for the 12-16 age group is teaching them to mod minecraft.

This is a little more advanced, obviously, so it's not a great activity for total beginners. However, once a kid learns that they can make exploding blocks just by writing about 5 lines of code, they're totally hooked.

It also gives them an initial idea as to what real software looks like. Once that is demystified, their confidence as coders increases substantially.