I went on Coral Princess, 10 day partial transit Panama Canal, December 2006.
Took the Gatun Locks/Portobelo Excursion. Was a big disappointment. We were told that only those passengers who booked an excursion could get off the ship. This excursion took us by air conditioned mini-bus to the Locks, where we only spent 30 minutes. I would of liked to stay there longer. Then a loooong bus ride to Portobelo. No place to stop for a bathroom or snacks, and the boring tour guide put me to sleep. Portobelo is just an old, run down building housing some Fort artifacts. The single bathroom was flooded, and we had to stand in 1 inch of water to use the toilets. We were greeted and continually harrassed by small, soiled children trying to sell us beaded bracelets. The area was slum like, without a shop for snacks for those passengers who were diabetic (and the ship continually warned us NOT to take food off the ship....that the Panamanian Gov't. would search us.....which they never did). Then a loooong drive to Cristobal where the ship met us.
I would suggest staying on the ship as it turns in Lake Gatun, then getting off in Cristobal to check out the shops in the terminal. Be warned, there are native indians there who are topless/barely dressed. This is normal for them and the people of Panama, but shocked some passengers. Make sure to buy a handmade basket by one of these Embera Indians.
I also went on the aerial tram in Costa Rica. It was really nice, but quite a long adventure. The lunch was a delicious buffet, the guides were very informative, and the rainforest was very beautiful. Do not expect to see wildlife, as they are nocturnal.
Hope this helps.
Have a great cruise!