I usually take about $20 US...$10 is for the departure tax, the other $10 in small bills is for fun money at the airport at departure (at Las Americas airport once you're in the customs cleared area they only take US money).

You can change your Canadian money into pesos at the airport, at Western Union shops around the country and at banks and most major hotels. I generally take a little cash then rely mostly on ATM's...12 trips and no problems yet! The Canadian $/Dominican peso conversion is usually around $1 = 10 pesos so it's pretty easy to figure out prices.

Have a great trip!