What are you into? shopping, outdoors, architecture? Just yourself (male) or are you bringing a significant other with you?
I'm in Victoria on the island and it is raining like crazy here which means it is likely to be even more wet in Vancouver.
My wife lived over there for several years so I can ask her for you. When we go over it is to shop for things we don't generally see her on the island like Ikea. I like to hit up some good Bike shops and MEC (Canadian version of REI).
If it is half decent out, a walk through Stanley park is nice. The art gallery is supposed to be nice , although I have never been.
Vancouver is pretty diverse in culture which means there are all types of great restaurants to pick from. Again what do you like?
If you have time, head up to Whistler. The drive up can be quite nice, and it is a great little village. I absolutely love it up there. Spend the day. Walk around the village, and people watch. It is a beautiful place.
Let us know what you are into, and we'll try to make a suggestion or two.
Have a great day there regardless.