You can access Head Hunters on disk 1 and they provide the best EXP and AP on the world map. That sunken ship is a good place if you morph the enemies as you fight. You get exp and ap still I believe. The best place though is the northern crater on the upper left path. If you don't have the w-item materia then you can steal elixers from two enemies there. Tonberry and the horse monster that shows up with the guy with rabbit ears have them to steal.