Hello, I would recommend you either of the following depends of your preference:
If you take a TAXI at the airport (probably US$30.00 for the entire trip) and ask him to take you to the Panama Canal (just make sure the driver to take highway). You will have time to do the tour over Miraflores Locks and also eat on one of its restaurants.
The other option is to go shopping at Los Pueblos Mall near the airport (but anyway needs a Taxi) for less than US$5.00 do not pay more, and you will find nice/ship stuffs to buy.
Well there are many options for you, depending what you are interested in.
There is a lot of history and beauty to see in Panama.
You could take a trip to Portobello and visit the black christ, you can see the old cannons and arcitecture from where Capt. Morgan tried to attack to steal Panamas gold. There are Cuna Indians there with beautiful molas and hand made jewlery. There are also fabulous restaurants where you can get fresh seafood for lunch.
You can go to "Los Pueblos" a re-creation of the many sights in Panama. It's a great way to get a condensed idea of the different sights in Panama.
Panama la Vieja is also pretty cool. The architecture is beautiful and there is a museam there too. You can also do some shopping across the street from there and you can see a lot of blue iguanas.
The city including the Golden Alter and the Presidents "White House" is also nice but not as interesting as the above mentioned.
Have a great time and maximize those 7 hours!