Opened in 1914, The Panama Canal is one of the modern engineering feats and wonders of the 20th Century. Seeing the canal is a must on your trip to Panama. The history of the construction is extremely fascinating and moving.
Miraflores Locks: Close to town, the visitors center at the Miraflores Locks is a large structure with roof top viewing of the canal, an interesting museum about the history and building of the canal, a theater featuring a short video in Spanish and English, plus a cafe. I suggest going early so you are there right at 9 am to make sure you at least see some ships in the locks, otherwise you might have to wait until 1 pm to see any action happening.
Museo del Canal de Panama: Located across from the Catedral in Casco Viejo, this is a museum dedicated to the history and impact of the Panama Canal. It is well-worth the visit, but get the English audio guide as all the signage is in Spanish. You can check out art exhibitions on the second floor. I just missed a Paul Gauguin exhibition.