I have a thing for lighthouses and love to collect photos of them.
I really enjoyed the Key West Light. When we went, we were free to roam around and read at our leisure. There was no formal tour. You can go up into the lighthouse, which provides some excellent views of the whole island and allows you to get a great aerial shot of the Hemingway House; my photo looks like a postcard. The spiral staircase inside can be a little scary if you are acrophobic, however, so be forewarned.
There are lots of nice displays in the keeper's cottage that will teach you about other lighthouses in the area, and a small gift shop on the property where you pay your entrance fee. I found $1-off coupons on the Internet that helped save us on our admission charge; I also saw such coupons in free booklets distrubuted in various boxes around the island--be sure to look in those if you plan to do alot of sightseeing.
I found a really nice snowglobe featuring the lighthouse at the Sponge Market near Mallory Square, so don't feel that you're limited to the souvenirs available at the gift shop.