Gatun, the coolest of the 3 Panamá Canal locks, is a 1-2 hour drive north from Panamá City to Colón, depending on how frequently you get lost. Driving in Panamá requires navigating by the sun and stars as there are few road signs. Approaching Colón there is a huge traffic circle and the key is to take the westward option to link up with the road to Gatun, and eventually signs start popping up for Gatun. Further along is the UNESCO-listed Fort San Lorenzo which makes for a great combo trip.
Gatun has no museum like Miraflores. What it has is viewing platform right on the top of the action, so close you can chat with the sailors passing through, and the train tugs that pull the boats at each corner pass right under the platform. Captivating.
For non-Panamians/Panamá residents the entrance fee is US$5.