You've already given us a gift! We recognize that making the long journey to Cuba for our wedding is a huge investment of time and money and we thank you so much for joining us for our happy day.
It depends on what you're using the money for: Tipping - CAD, USD and Euros are appreciated (note that Cuban's can't take looney's and toonies to the bank so consider using bills for tipping) Buying things at the market - CAD, USD and Euros are most commonly accepted. You'll get the best deals with USD. Restaurants - you'll usually see prices listed in multiple currencies on the menu (and they don't usually correlate with official exchange rates), if Amed and I are with you, you should check with Amed to see which currency will give you the most bang for your buck at that particular establishment.
You don't need to. And...it can be difficult to convert money back into CAD. You will not be able to convert USD into pesos at an official bank/cadeca.