Burn a hole in someone's newly paved asphalt driveway using thermite.
-Place industrial strength smoke grenades (the sort that will fill up entire buildings) in obscure places in a public building. Also good in someone's car or truck.
-Coat a black toilet seat with a *very* thin layer of nitrogen tri-iodide. NTI is an easy to make contact explosive that will leave a purple stain on their rear. Do not use very much or you will hurt the victim.