A derndytrek is an excursion that stoners go on when they get high which has no particular destination or purpose. Derndytreks typically involve exploring places where the stoners have never been before, even if such places are close by. Derndytreks can be done on foot or in a vehicle.
Stoner 1: "I'm bored. Wanna get high?"
Stoner 2: "Yeah, let's take a derndytrek to Pelham Bay Park. I've never been there before."
Stoner 1: "Ok, but let's not get lost in the hood like we did last time."