Daren, you dock directly across from the fort, so you can easily do that on your own. My niece, however, said that the taxis and sales people are very aggressive, and that if she did it again, she would have booked a formal tour.
I went on the Cultural one, which gave you a great overview of the city -- stops at the cross at the top of the hill, thru Las Brisas, Diego mural stop, past some old celeb's homes, and to the Flamingo Hotel for a beer ... oh and also the obligatory "shopportunity"
Harry