The AC flight to Sydney is probably one of the single most in-demand routes for Aeroplan international redemption. J seats typically go the day they become available 350 odd days before departure.

AC does not fly to Cape Town - you need to connect with a Star Alliance partner in Europe or the US. I suspect this one is an extremely popular route within Star for redemptions as well.

Having said all of the above, I'd suggest calling Aeroplan. Their on-line booking engine is atrocious at showing award availability for Star Alliance partners. They'll ding you $30 per person when you book, but I suspect you will be able to find J seats to Dubrovnik and possibly the other destinations if you have some flexibility with your dates and/or are not going during peak travel times.