Potassium permanganate, even in these over-regulated times when perfectly sensible six-year-olds cannot buy arsenic over the counter, is still pretty easy to find in most allegedly civilised countries.
If you've got your hands on some of those pretty purple crystals, and also have some ordinary supermarket glycerine, and you pour the latter onto the former...
...this will happen.
If you'd prefer your spontaneous combustion with a more traditional audience of stoned-sounding high schoolers, this second video may be more to your taste.
The reaction will occur faster when the permanganate crystals are smaller. The smoke has a pleasantly firework-y smell, and is not a lot more toxic than you'd expect any other smoke you found in your kitchen to be.
This reaction is, of course, easily adaptable into hilarious devices for setting fire to school rubbish bins, automatic teller machines, ballot boxes and so on.
I also remember substituting potassium permanganate for potassium nitrate - of which I didn't have any - in gunpowder recipes. The result was a nice flammable powder, but of little use for making weapons of mass destruction.
This all works because potassium permanganate is quite a strong oxidiser. Not "quite a strong oxidiser" in the charmingly understated terms of the kind of chemist for whom anything that'll leave the paint on the walls of the lab clearly does not even justify the use of eye protection, but still strong enough to spontaneously react with numerous other common chemicals. Glycerol just gives you the best bang, or at least whoosh, per buck.
And, again, potassium permanganate is quite pleasingly non-dangerous. No, you shouldn't feed it to your toddler, but it's only moderately toxic.
Oh, and it's about the most intensely purple substance in existence. One small crystal will slowly and interestingly purpulate a large bottle of water. And that water will, then, leave yellow stains on things. Just to keep it interesting.