One of the most important things to note is that U. S citizens don't need tourist visas to visit Panama but in this case, they can only be in Panama for up to 180 days.
Other necessities include
At least $500 in cash or on your bank but only up to $10000 in cash.
A return ticket back to where you came from or the next destination.
For minors: Parents' IDs and minors birth certificates for Panamanian citizens ( even if minor is a dual citizen). Minors who aren't Panamanian citizens are exempted from this requirement.
An entry fee of $110 if you are entering by sea.
Note: This fee only allows you a stay of up to 3 months but it can be extended for 2 years at most.
Visit the embassy of Panama website to learn of any changes in travel requirements.