The Space Centre is located in Vanier Park; you can take bus #22 straight there from downtown. It's then about a 5-minute walk from the bus stop; follow the signs and head for the domed building on the green.
The Space Centre is actually part of a complex with the Vancouver museum, and you can get combined tickets to both. You also have a choice of tickets for the Space Centre, and these can include entry to the International Space Station display, the VR rides, the planetarium show, and the basic space exhibits.
The main exhibits were quite fun. Some were interactive, and most were aimed at children, of which there were hordes. Try to get to the planetarium show early to avoid the aforementioned kiddies. The show itself was quite interesting, focusing mostly on the development of astronomy for a change.
The VR trip to Mars involves you in a life-saving space mission as well as the special effects and the bumps.
I most enjoyed the International Space Station display, which included a life-size replica of the station and all its gadgets. At the time, the ISS was passing directly overhead Vancouver, and I was able to spot it in the night sky, which quite impressed me.
If you fancy a walk in Vanier Park afterwards, the park is pretty unexciting-- a flat stretch of green alongside the water--but there are a few picnic tables for a sunny day.