If you fancy some brilliant dim sum, head to any branch of Tim Ho Wan, or try One Dim Sum (which I think is slightly better than Tim Ho Wan). For pretty much the cheapest Michelin-starred food in the world, you can't go wrong.

If you take the Airport Express to Central, take the Star Ferry over to Kowloon, MTR to One Dim Sum, you'll still have time to spare for a drink or two before taking the Airport Express (from Kowloon this time) back to the airport. A day return Airport Express ticket is (I believe) a very good value.