We walked to the old town centre from the port in about 20 minutes but it's worth the walk. Don't waste your money on an excursion as we strolled around the same places as they did. Bari is a lovely city and well worth returning too for a couple of days.
We were going to take an excursion to the Olympic site but a lovely Irish couple told us for 10 euro you can take a bus just outside the port and they'll have you back before the ship leaves. However, we ended up taking the train (6 minutes walk from the port but it's a very small station so ask someone where it is).
The Olympic site was a bit disappointing as it's a bunch of fallen rocks and a lot of imagination but there isn't much else to do in Katakolon.
The walled city was fabulous. We caught the local bus from the port which cost a couple of euro. We walked half the wall then had a lovely lunch and stroll around the old city before going back to the ship.