Festung Ehrenbreitstein is the biggest attraction, but unless you have tons of time, it is only worth the pretty view.
The best part of the excursion was trying to find how to get to the Sesselbahn (chair lift) to get up to the fortress. We took a ferry across the Rhine (DM 2, one way...about US $1) and then hunted for signs. No signs...but it is easy enough to see the fortress, so we walked toward it. There is a tunnel with an arrow and word 'Sesselbahn), nothing more. We walked into this empty, gloomy tunnel and giggled nervously about how dangerous it is to just follow an arrow. Finally reaching the lift, we headed on up to the fortress, which was closed. The view is pretty, but not spectacular.
The castle is nice but the view from the castle is another aspect of the castle that should be enjoyed.