If you booked a place near Douarnenez, I would recommend visiting the lighthouse in Penmarch, they are open all year round (more info on www.penmarch.fr), it should be a quick drive from Douarnenez.
Also don't miss the "baie des trépassés" (the bay of the dead) close to the Pointe du Raz.
Moreover, if you want to see some Vauban forts, head to Concarneau, only 1 hour away from Douarnenez.