centre island is great. of course, centreville is lots of fun for the kids but besides that, it's huge and there are lots of nice spots. lots of expansive grassy space for picnics, playing games; bikes for rent (or you can bring yours across on the ferry); a little "tour bus" to show you around (only $9 for a 35min ride); wonderful beach spots, especially out by gibraltar point...
it's $7.50 for the return ferry trip. on busy days, i recommend buying your ticket online ahead of time so you can bypass the lineups at the docks.