Is a 9-hour layover too little to see the Canal? Otherwise, what recommendations do you have? Thanks!
It all depends on which 9 hours. We really can't give you decent tips and ideas unless we know what day and times you are arriving and departing.
This would be a Saturday in June from about 6:30 am to 3:30 pm.
You could see the Canal. Get a taxi to the Miraflores visitors' center. They open at 9 AM. It will take you about 45 minutes to an hour to clear immigration and customs, then a 45 minute taxi ride to Miraflores. You could stop for breakfast on the way. If you want to try Panamanian food, have the driver take you to Manolo's Café on Via Veneto. (It's good and it's cheap.) The taxi ride from the airport to Manolo's will be $30. From Manolo's you should be able to get a taxi to Miraflores for $7 to $10.
The visit to Miraflores will probably take you about two hours. Go to the viewing areas first, as they stop ship traffic through the locks between 10 and 11 AM. Then, go through the museum. By then, it will be around lunchtime. You could head back to Tocumen, or you could stop for lunch in Casco Viejo or along Av. Balboa. Assuming you don't have bags to recheck and you already have your boarding pass, make sure you're on the way back to the airport no later than 1:45 PM.
Thank you so much for the outstanding information!