I'm going to be in Vancouver, BC for a day trip. What do I HAVE to see?

I’ve never been lucky enough to get to Vancouver, but have personally always wanted to visit their gardens

An excellent start!

Here’s a clear outline of a perfect day tour in Vancouver. You do need a car.

http://www.tourismvancouver.com/visitors/things_to_do/perfect_day_itinerary

Stanley Park is a must see, although one day cannot do it justice.

The Sun Yat-Sen Gardens are very nice,

I have to agree with @marinelife that Stanley Park is a must see. You could also stop by the Vancouver Public Library and see my friend, Kay. She is the virtual librarian and a delightfully interesting person. My sister and I took the Victoria Clipper from Seattle to Friday Harbor and the San Juan Islands and it was a wonderful time – cold in June for Carolina girls, but we had fun!

You could easily spend a day in downtown Vancouver. Stanley Park is just a walk away from many historic sites. The Vancouver Art Gallery is just blocks away from the Burrard Inlet. You could beach comb Kitsilano which is a cab/bus ride from Stanley Park. There are many shops and great restaurants all through those areas. I personally love the Maritime Museum. The Saint Rock is housed in a really neat Maritime Museum. Vancouver has a vibrant Chinatown.

I hope you have fun and sunshine.

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.