I’d have to concur with gailcalled on the linked itinerary if you have a car and don’t mind driving as much as it would be. That trip is a very effective way to compress a lot of the area into your day. If that doesn’t appeal and you’d want to visit some specific areas, you can easily spend a few hours at the Van Dusen botanical gardens, which I attend yearly. Gastown is really a historical must-see, and the architecture is quite wonderful as well.
If you’re the outdoorsy type, but again, the driving doesn’t appeal– I’d recommend the Grouse Grind to stay local. It’s a phenomenal workout with some 2830 steps. Once you reach the top, you’re greeted by an incredible vista of the lower mainland and the Ocean. From there, you can attend the restaurant for a fantastic afternoon/evening meal, and ride the gondola down for another fantastic view of the mountain.