Buses leave from the central train station usually between 5:30 and 23:00 and will take you to most parts of Koblenz. The city itself is quite small and easily accessible on foot.
You can reach Festung Ehrenbreitstein by taxi, bus, or by taking a ferry across the Rhine River and walking up the walled access road to the fortress. You can also ride a chairlift Sesselbahn to the top of the fortress.