The canal is nearby (you can see it when landing) and was recommended to us as an interesting site to see during a layover. We didn't do it for various reasons so I can't comment, but it looked doable and appealing.

There are also beaches, jungles and downtown Panama City within taxi distance, though given the country's developing world status (e.g. the airport's power went out just as we arrived) you'd have to be pretty confident in your travel skills to go there.