For $100 CDN you will receive approx. 72 CUC. First thing to do is tuck 25 CUC into each passport for your exit visa.
I haven't been to Varadero or Havana but in the Holguin area a dinner with drinks costs about half of what you spend at a 'Mom & Pop' place at home. (not Mom and Pop's place, but a restaurant!).
Excursions are best put on a credit card, Canadian banks only. No US affiliation cards are accepted, check with your bank.
Some hotels have a money change booth where they charge a transaction fee. Change a few $ more than you think you will spend and you will be ready for the 'duty free' at the airport on the way home. If you have too much left over, someone here at home will be happy to take it off your hands for posted rates.
Anything else? Fire away!