1. Air traffic control. Given the importance of guiding aircraft to and from their destinations, and the relatively exposed nature of them currently, more could be done to keep them secure. It would be relatively simple to destroy one completely with an RPG fired from a van that drove close enough in a parking lot or access road.
2. Power plants both nuclear and conventional. Everyone is afraid of nuclear plants being destroyed, but look only as far as the disaster in Tennessee several years ago when the slush pond dams broke and millions of gallons of toxic waste spilled out destroying the land for generations.
3. Though it isn't a specific target to be hardened, controlling access to firearms would be a great place to start but likely impossible.
4. Key bridges and tunnels that allow access to NYC, Baltimore, etc., are currently almost totally unsecure.
5. Civil air fields are also almost completely ungaurded and give terrorists access to smaller planes that can be used to deadly effect as we've already seen.