Maybe try Avis Car Rental
at the DoubleTree by Hilton Hotel Denver
. See if you can use the hotel airport shuttle.
Or Enterprise Rent-A-Car
is at the Courtyard by Marriott Denver Stapleton
You may be able to find a deal on airport car rental if you search all the different travel booking sites, so don't give up on that option. You'd still take a shuttle from Jeppesen Terminal, DEN
to the airport rental agencies. See if you can rent at the airport and return downtown, or vice versa. Sometimes that might save you fees.