The Public Market is arguably the best place to spend a Saturday afternoon in Vancouver. Get there from downtown via the two private ferry companies (False Creek Ferries/Aquabus).
Great options on Granville Island (not just the Public Market)
-If you're looking for a date option, check out the Vancouver Theatresports League on Half-Price Thursdays (vtsl.com). Cheap hilarious comedy.
-Check out the Granville Island Brewery Tour/Tasting. They have a great seasonal rotation, so you always get something new.