Hi Karen,
You can pay for almost everything by credit card - make sure you have one that doesn't charge that extra fee for foreign exchange transactions.
Vancouver is close to the border so you can actually tip in US$ if you don't feel like changing money. We go to Canada quite frequently to visit my in-laws and I have to confess we hardly ever change money any more - we just use plastic and American money. It all depends on what you want to do during the day you are there. If you are going to use a taxi you might want to change some money.
Vancouver is a great walking city. Stanley Park is lovely with great views. The shopping is cool on Robson street and also on Granville Island (lots of art galleries and funky, crafty stores and a public market). Gastown and Chinatown are also good to wander around in - just depends on what you'd like to spend the day doing.
Have fun!
Mary Ellen