The observation deks on Terminal 1 should be open. The one on Terminal 2 is closed IIRC. If the weather is lousy you can watch planes from Terminal at the A-Gates between A 28 and A 40. This is a relatively new concourse add-on with gates on only one side of the concourse and glass walls that face towards the runways on the other.

My recommendation, though, is to either go to Mainz as tom911 said or to Wiesbaden. Both cities can easily be reached by train.