As many times as I have done the Panama Cnal Cruise I have yet to book an excursion.
Have heard good reviews on many of them however.
As for the soda card or sticker as it is -
You pay a set fee at the start of the cruise for unlimited fountain soda. The price varies based on the length of your cruise. But, usually low $20s.
A can of soda costs approx. $1.75, so if you are a big soda drinker the sticker is well worth it.
Another tip, ask for the large tumbler glass when you go for your drink. They will try to sell you a small sized cooler cup, does not hold much soda at all.
The Coral is a great ship. I am sure you will enjoy your cruise.