Crystal growing kits (like teacher camp activities)

I really like that nearly perfect cube of NaCl (Does anybody know why would you need KCl in the solution?) I may make that a task from one of my student aides this year.

Or I may try a few different crystals that come in a crystal growing kit (item 15114) from Flinn Scientific.

Every year in our material science course, the students grow copper (II) sulfate crystals, so I'm familiar with the pretty blue crystals (shown above in the video, too), but I haven't tried the other chemicals (alum, chrome alum, nickel sulfate, and sodium chlorate) that come in the kit and which, I assume, grow via the same procedure. I should know what they look like and will post photos after this year.

Yes, I'm quasi-endorsing the purchase of a Flinn lab kit (though not this one). They're a nice way to get and try a lab once, but I will fully endorse buying the resupply chemicals separately because the kits are always more expensive than the raw materials.