We spent 5 hours at the museum and that was way too little time. We started with the imax movie about the Hubble, and then went up in the observation tower (hint: don't stop at the 6th floor for the air control exhibit; it's not nearly as interesting as the rest of the museum and you can get stuck there for a long time if the elevators are crowded). We wandered around the main floor for a while, but the most satisfying part of the day was when we connected with one of the free tours. Elaine, our tour guide was a retired air traffic controller and was very well informed and entertaining. We had to leave after 1 3/4 hours and it seemed like Elaine could keep going for another couple of Don't be discouraged if it looks like you got there too late for a tour; they seem to leave on an hourly basis even if they are not posted on the board.