Drink with the fishes.
Why locals love it
A night in the aquarium. No children taking all of the costumes. Booze. And, of course, sea creatures! After-hours at the Vancouver Aquarium are loved by Vancouverites (19+). The Aquarium is loved by virtually everyone, and you can now play with the creatures in touch-tank without feeling like a weirdo (since touch tanks are for kids...right?).
Why you should visit it
If you haven't checked out the aquarium in daylight hours, and an event is happening during your visit, make time to check out these events (they happen monthly, or bi-monthly). If you think the Aquarium is something you should pass on your trip, my best advice is as follows: don't. Most people don't expect to love it as much as they do!
Buy tickets well in advance. They often sell out very quickly. See the webpage for details.