Mining job for idle worker

A cave or mine for idle worker to do some job. Once they are done, the result would be a chest or an item. The idea come from a digimon game where you paid it to drill the cave for certain days (either 3 or 7 days) and will give an item with certain rarity, depends on the job choice. Another way to spend gold and job for worker when you think no more new tower needed. ?