Depends how fast you need it. AVIS transactions post fairly quickly (my August 31 rental posted in September) so if you rented a car at a weekend rate (about $30 CAD for 500 miles if you rent off airport, all taxes in). You could do two rentals in one weekend (e.g. Friday and Sunday)--check out Websaver specials or some of the fall promos on now, I think AVIS does get Aeroplan points on Websaver specials, Budget's do not. Other than that, I can't think of how you might quickly do it unless someone in your family or friends is about to make a major purchase--in which case you can use your Aerogold card to make it for them.

Make sure the Car Rental Agencies have your Aeroplan number when you check in, also give it to them when you check out just to be sure.

You didn't say which award level you were trying for but it sounds like you might want to think about using real dollars to earn more points though (ie. fly on one of AC's cheapie fares to where you want to go for this reward trip).

Keep on earning points until you get enough to get real value out of your reward travel. Just my 2 cents.