We aren't leaving until 4/5, but we booked the SanLorenzo & Gatun Locks excursion in Colon, Panama as we had heard great things about it and didn't want to miss going through it. We also just booked the Xunantunich Mayan Ruins as it is supposed to be one of the best of the area. It is a 7 hour tour but figured it was worth it when comparing the choices of the two offered.
We have not picked anything in Costa Rica as of yet. Wasn't quite happy with the choices without getting more info on them.
If you decided to go diving, just do it through one of the ship excursions so that you are comfortable as they are safer than do one on your own in that particular area. I have been there a few times and just need to be careful, like any other foreign area (or big city here)!
Hope this helps and please email me when you get back with any updates or comments, would love to hear form you!
Have a great time