As a United 1K I love Hertz’s 2,750 miles per 1-day rental, especially since I find the United rates competitive for many of my rentals. Without United status, the earn is 2,000 miles.
Air Canada Aeroplan has a great offer through December 31, 2016 for 4,000 miles per 3-day rental. A 1-day rental earns 1,000 miles. This is for rentals in the US, Canada, Mexico and Puerto Rico.
You need to reserve with Aeroplan CDP# 221417 and PC# 111075 so always compare to rates with other discount codes. Details here.
Aeroplan is a strong value Star Alliance program. Look for international tickets on airlines that are without fuel surcharges. Domestic Canada and US-Canada flights typically have poor availability and high cash component.