HI Sassifraz, the horse and carriage ride place is located in front of pier #1.
They have a short middle and long ride, the short one is about $35 per couple and ??? $$ extra for additional information.
If that doesnt work would you consider a drive around old San Juan.
We used Tito at email@example.com
he is a local certified tour guide that knows his stuff, checking him out and have a great cruise