Glad you asked...
You can use some fruit juices to make a dye that will work like henna. Try using raspberry or blueberry juice to make a stain for tattoos.
Then again, this answer might help you...
There is a plant called Jagua that a natural dye can be pulled from. This is a safer substitute for Henna powder. You can take a trip to South America to gather the plants to make temporary tattoos but an easier route would be to purchase it in gel form.

