Hi - you should be able to bus (or walk) from Robson to the Sea-Bus terminal (near the cruise dock) - catch to Sea Bus to the North Vancouver side then hop a "Lynn Valley Road" bus to the ecology centre and suspension bridge. I grew up in North Vancouver and, while the creek was always there the bridge is fairly recent. :o)
If your ship has sailed - swim out to it.
We have the same time in Vancouver as you. Although we're doing all our own exploring once we get to AK, I opted for Landsea tours in Vancouver. They pick you up at your hotel, give you a 4 hour tour, and deliver you to the cruise terminal. You can check them out at www.vancouvertours.com (Grayline also offers tours, but Landsea was recommended to me as being more personal.) Just another option to consider.
We booked LandSea Tours when we were in Vancouver in 2006. We arrived in Vancouver a day before our cruise. LandSea Tours have tours of varing lenghts. We took the Capilano Suspension Bridge and Grouse Mt. tour. We arrived about 1:00pm and they picked us up at the Hotel about 2:00. Very knowledgable tour guide, very professional, and our guide was alot of fun. You also can book on line. We did, but I followed it up with a call . We highly recommend LandSea Tours.