The Long Island Railroad (LIRR) is running a summer beach package to Kismet and the Fire Island Lighthouse from May 24 to September 1. They have a deal with David Bros. Taxi so a round trip including taxi to the ferry is $34 from Manhattan. This fare also includes general admission to the Fire Island Lighthouse. More info can be found here:
http://mta.info/lirr/getaways/BeachPackages/Kismet.htm
Don’t forget, you cannot ascend the steps of the lighthouse with open toe shoes, so remember to bring sneakers!