There are products you can buy that claim to add some flavor, grape or citrus or berry. They kinda work, they change the flavor but I'm not sure if its for the best. A few weeks ago a read a post where the poster had added vanilla extract to his res 24 hours before harvest. He said within 2 hours he could smell the vanilla in the buds. He had been flushing for a week or so, and he was in hydro. I think your in soil, but the principle is similar. 1-2 days before harvest you could mix some vanilla with water and water your plants. I like when my weed smells like weed, and nothing else.

Its pretty safe to add sage water during the last 2 days. If something bad happens you just harvest and hope the flavor is good if there is any at all. When I say bad, i mean drooping, wilting, color change, instantaneous combustion.
I once wanted to add food coloring to my res to make some crazy colored nugs, but I got stoned and forgot.