Once the ship has climbed the three Gatun locks into Gatun Lake, it will tender those on shore excursions into the Gatun Lake Yacht Club where they will board their various means of transportation. Those that are not on the ship's sponsored shore excursion cannot get off the ship.
In the afternoon the ship will go back down the Gatun locks and cruise to Cristobal Pier where the passengers that took shore excursion will be picked up. While there is shopping in the cruise terminal, there isn't much more to do at Cristobal Pier.
The bottom line is that I would suggest signing up for one of the ship's shore excursion for the day.
I hope that helps.