I've sailed on Mariner and transited the Canal, but not at the same time. My advice is to get up early and grab a chair in the air conditioned observation lounge. On deck it is hot and muggy, not conducive to standing out long.
Mariner is a terrific ship and I'm sure you'll love her!
I made the same trip on Seven Seas Voyager. Radisson goes through the Canal differently than other lines. You will go through Gatun locks very early. Get up before dawn and watch from Deck 12. Then, head to La Verandah, take one of the tables on the stern and have breakfast while finishing transit of Gatun Locks. You will then stop at Gatun Yacht club. There they put on some entertainment and have a nice bbq. One thing that is neat to do there is to actually swim in the Canal. If you are interested, they do have other excursions. At 1600, they will again start the transit of the Canal including the other two sets of locks. It is kind of neat going through the Miraflores Locks after dark.
One other must do is the Canopy Tour in Puntarenas. You go down a series of 11 zip lines. Lots of fun.
I went snorkeling in Cabo but did not go ashore the rest of the trip.
Seven Seas Mariner is a fantastic ship. We sailed her for 19 days and could have stayed on forever. I don't know your ages, interest, or cruising desires so it is hard for me to anticipate what more you want to know. Let me know any specific questions you might have.
Thanks for all the good advice. I am 47 and my wife is 45 and we are looking forward to both swiming in the canal and the Canopy tours. Not sure we will do the zip lines or just the walk across the bridge. We have been on the PG in Tahiti and love RSSC so we are looking forward to the Mariner