You are much more likely to encounter a last minute contract rider if you look like you are going to Burning Man or say you are going to Burningman when you pick up the car. More likely to receive extra scrutiny on return, potentially activating extra fees if you are dusty and the car is dusty.
Playa dust is extra wear and tear on the vehicle and if it gets in the ventilation system and carpeting, it will irritate any future renter. That's why I never take my personal vehicle! You have to realize it's a risk and cost to the rental company.
I would be vague, "friend's wedding, family reunion, Las Vegas, Burningman, what's that?". Detail the car on return. Cover the seats and cargo area with old sheets, cover the dash when parked. If it's not in the contract, it can't be enforced.
Even so everyone who says, "I'll just screw the rental company, so what it's trashed, or I'm to wasted to deal" simply screws hundreds of next year renters for the event.
increasing the signal to noise ratio with compassion