I can imagine the discovery of hashish pretty easily.
Think back 10,000 or more years ago, even before your people invented agriculture. You run across a big field of cannabis' wild ancestor. It's autumn and the seeds are ripe begging to be eaten. You know about the value of seeds, all kinds of seeds, and collect them whenever possible. In the process of pulling seed heads and stripping away their sticky outer bands of protection your hands get coated with sticky resin. You lick your fingers.
What I have more difficulty imagining is the invention of smoking.