http://www.sunbeamfleet.com/lighthouse_cruises.html This is a great lighthouse cruise out of Northern CT. if you can work it into the right weekend.
http://www.soundnavigation.com/ This is also in Southern CT. Worth checking into. You can also get to Shefeild Island.
Great Captains light also now allows anyone to use the ferry and go see this light. You will have to wait till they post for 2010.
If you have time Block Island for the day is fun. And most of the lights are easy to get to. Dont miss Castle Hill in RI. There is also a boat ride that does most of the lights http://www.rhodeislandbaycruises.com/Lighthouses.htm
Lots of options and they are all great.