As I see it, three factors should be in play here:
1. Location - a place where most people would want to go and have facilities to accomodate a large number of people.
2. Expense - the cost has to be reasonable so most people can afford to go.
3. Lighthouse density - the location has to have a decent number of lighthouses in the vicinity to visit.

So...
Hawaii - fantastic location, but expensive and I am concerned about the lighthouse density factor here.

Alaska cruise - same thing as Hawaii...I'm not sure how close a cruise ship can get to some of the remote locations.

Great Lakes region - Michigan would be ideal but I get the feeling that finding an area with adequate facilities may be an issue (is this correct?). Chicago would seem to be a natural choice too. If we look a little outside the US, there is Toronto which has a lot to offer as a city and is close proximity to alot of lighthouses...but perhaps too much "east" for the west coasters who have already had to travel east twice.

Florida - Miami or Tampa would probably be the best choices here. Again, the west coast factor might again be an issue here as well.

West Coast - San Francisco or Seattle seem to be the natural choices...they are the right size and are nice places to visit. They also have alot of lighthouses in the area.