My wife and I (54 and 57) walked from the ship to the cliff divers at night no problem. It's a bit of a hike and the last part is a bit uphill, but if you like to walk it's great. There is a fee to watch the divers.
Also, at night you don't need to fend off as many of the "tour guides" that hover just outside the cruise terminal gates.
During the day we walked along the beach, going counterclockwise around the bay. Ended up turning around by a little light house, at a swimming area popular with the locals. But along the way, we passed through what seemed like a little Mexican fishing village that exists 100 yards inland from the local fishing boat docks. Really nice canopy of trees and a small fresh fish restaurant. It didn't seem like we were in a city at all; it was a totally different experience.
The old fort and museum is right across the street from the cruise terminal.
Don't forget, walking helps burn off all those calories. We ate plenty on an 11-night cruise, and I didn't gain any weight at all. The secret was to get off the ship in every port and walk, walk, walk.
I would definatly get a cab to the cliff divers. We were there last year on the Star. I would recomend going at sunset. It is even better watching them jump in the dark and the last diver jumps in with a lit torch. If you get lucky like we did, your cab driver will wait for you to bring you back. There isn't a lot of cabs to pick from to come back on.
Celebrity Mercury - Hawaii
Princess Golden - Meaxican Riviera Oct/Nov '07
Norwegian Star - Mexican Riviera Oct '06
Carnival Ecstasy - Catalina/Mexico April '02
Disney Wonder - 3 day Bahamas Nov '99