Incredibly scenic ride from Vancouver up to Whistler. Takes 2-3 hours depending on traffic and construction. Even if you don't ski they have other activities. If they have tubing (I'm not sure but they must), the kids will go insane. It's tons of fun.
Stanley park is nice, but short of a warm day and a long picnic you'll be in and out pretty fast. Do it, but don't expect to spend a day there.
Cross over to Lions Gate and head up the hill to the capilano suspension bridge. It's pretty cool - kids will like it. I was blown away by the size of the trees. They have suspended walkways that go from tree to tree. Reminded me of planet Endor.
If the weather permits, rent bikes (cheap) and ride along the river - great bike trail.