For popular tourist type destinations, during peak tourist seasons (i.e. Winter break, Spring Break, Summer vacation) you should plan on booking up to 1 year ahead for Aeroplan seats. I booked the first week tickets were available (April this year) for Maui in March 2003 and all seats allocated to Aeroplan points redemption were gone in a week (except for a couple unusable combinations).
You should be aware that for each class of fare, including aeroplan award seats, Air Canada will only allocate a particular number of seats with varying restrictions depending on how much the customer is willing to pay and the need for flexibility. Empty seats does not mean that any aeroplan seats are available. There are also black-out dates for certain destinations (including Florida and Hawaii) that will further restrict your attempt at award redemption.
The only alternative may be to try booking on a United Airlines flight through aeroplan if you haven't tried that already.
Basically, you are stuck with the Aeroplan rules and don't get much more choice unless you are an extremely frequent flyer and qualify for the 'higher' levels of aeroplan membership that permit some bypass of the seat restrictions.