When are you thinking of coming? There is nightly entertainment all thru July/August at the gardens. Also, fireworks on Saturday nights. It's fairly close to the ferry about a 20 minute drive. From downtown - where the seaplanes land it's probably about a 45 minute drive or more depending on traffic. The ferry ride is beautiful and the ships are fairly nice.
Here's a few url's for you.
http://www.butchartgardens.com
http://www.bcferries.com
There was a company last year offering a passenger boat service from downtown Vancouver to the gardens. I'll need to dig around and see if I can find that info for you.
Also, another option for Vancouver is Granville Island Market. Real interesting place to visit - farmer's market, unique shops and restaurants. Fun in the summer.