Enterprise Rent-A-Car offers a freedouble upgrade when you book a rental online and pick a participating locations. (Minimum 1-day rental with a Saturday night required, with a 4-day maximum.) It's the best such upgrade offer we've seen from Enterprise in over a year. Fees, insurance, and taxes are additional and may be substantial. It's valid on advance reservations made for Economy through Intermediate size vehicles. Rental period must end on or before November 14.
Note that your upgrade won't show on your reservation; bring it in to your rental location and they'll give you the bigger class as available.