Quasi legal booby trap;
Take 1/4 lb of Iodine crystals and dissolve in a quart or less of Ammonia. IT IS Safe while wet with Ammonia, Optional - add a couple grams of powdered potassium or magnesium.
Apply wet brown sludge distillate to back of sign and allow to dry. Do not touch! If touched there is a big cloud of purple smoke and resounding blam. Even a blade of grass could set it off.
Or be creative;
For small blams pack keyholes with wet sludge. Splattering wet sludge with paint brush on basement stairs and walls will create spontaneous bangs when dry for days or more.