I've had a character in the past who would have the party wizard cast explosive runes on strips of parchment during all the downtime the party had and roll them up into neat little scrolls. My character (A scout) would tie the little scrolls to arrows and sneak around camps shooting the arrows into tents and whatnot while the enemy camp slept (not targeting people or things that can be loud). Then, presumably, the enemies would find the arrows with little messages, open them and cause chaos in the camp, taking out structures, people, and resources throughout the next morning while they were preparing.