One of the best cheapest things to do in Sydney is to take the Manly Ferry from Circular Quay especially if it's a fine day. It takes 45 minutes one way, and is much better than booking any dinner cruise. Once you get to Manly, there's not that much to see/do except maybe walk to the beach, but the beach at Bondi is much nicer, so I generally turn around straight away and get on the next ferry back.