Aeroplan launched interesting Star Challenge last week where you can earn up to 16500 bonus miles by completing number of transaction using their partners. Different partners/transactions earn different number of stars. This promotion started on May 1st and ends on May 28th, 2012. The valid promotional period is really short.
There is no need to register for this offer. It is valid for only Aeroplan members that have their account address in Canada. If you happen to reside somewhere else, but have a friend in Canada, it probably would be best to find out if you can use their address.
There are quite a few ways to earn Stars rather cheaply. I am sure that you can figure the ways by having a look at the partner list.
35 Stars will earn you 1500 bonus miles
70 Stars will earn you 16500 bonus miles
The number of stars earned is buried on the Terms and conditions of this offer.