Nothing attracts boy’s (including overgrown adult ones) attention like playing with fire. One of the easiest and safest ways to make a fun fireball is to use an unconfined mix of a flammable powder and air. Essentially this is an explosive aerosol so don’t confine it. It is a great deal safer than burning hairspray or insect repellent, and probably would pass the guide to safe scouting. (Both of my son’s troops ban aerosol insect repellent because the boys have discovered how “good” a flamethrower it makes – the powder isn’t flammable until it is in the air so you can’t have either a continuous flame or backflash into a can which could turn it into a grenade).
Following up on a mythbusters episode we decided to try powder dairy creamer. We built a couple of fancy devices with plastic tubes and a candle, and while these worked the easiest thing is to put some on your palm and flick it into a flame.