Welcome aboard, Lisa! I bet you are very excited about your upcoming cruise on the Fantasy! It's a beautiful ship with so much to offer as far as activities. You are going to have an amazing time!
Most of Disney's
Port Adventures do leave shortly after the ship docks at the port, so that the adventure guides can make the most of your day there. That being said, if your Port Adventure is not more than say four or five hours, you will certainly have time to explore the port area once you arrive back from your excursion. The longer adventures are the ones that might have you out all day, not arriving back until a few minutes before you need to be back on the ship.
Either way, by choosing a Disney-sponsored Port Adventure, you can guarantee that even if it is running behind, the ship will not leave without you. This can't be said about booking your own excursion through a different company, so for us, it's worth the cost to have that peace of mind.
Most likely, Lisa, you'll arrive back from your excursion with enough time to wander the port area and pick up a few souvenirs before getting back on the ship. Just keep in mind that lines can get long to re-board, especially within an hour of when the ship is set to leave, so watch the clock to ensure you don't miss the boat!
Have fun planning, Lisa!