How about caravan raids?
Caravans containing random resources and gold periodically cross the map, perhaps with a few guards and you send minions to kill them. Caravans should have a nice BIG cache to make it worth the players while to raid. Also, should only be detected if you have a creature like a crow or something nearby, in order to make it worth your while to have scouts. BUT … that means the reward should be higher.
Of all the suggestions this sounds the most FUN