Nassau is not my favorite port, we walked to the Queens Staircase and Fort Fincastle ($1 entry fee), be aware that the streets do not have the best signage, it took us a while to find it but it was really pretty with amazing views. There are "tour guides" who try to corner people to get tips, walk past them if you can.
We took a taxi to El Morro ($15) and purchased entrance to El Morro and San Cristobal forts ($5 each) the walk to San Cristobal takes about 15 minutes, both were stunning the views were spectacular, we both love history so we spent about 4 12 hours exploring the forts the 17th century graffiti was cool. Make sure you wear sunscreen and sturdy walking shoes some of the steps and hills are pretty hairy. Had a fabulous day for $25!