I came up with several varieties of field trip ideas:
First, you could check with your local science museum to see if they have a meteorology exhibit. If not, check with your local news station and see if it is possible to watch a taping of the news and/or meet with the meterologist or tour the meterology department. You can also check with your local colleges and see if the meteorology professor (most colleges offer at lest a basic weather course)would be willing to meet with you.
Second, you could tour your local American Red Cross and/or other local disaster relief agencies. Fire departments are another place that often respond after a disaster. Ask them how they help the community other than fighting fires.
Third, if you are close to an area recovering from a storm (they can take many years to recover from), you can find out a way to help the recovery efforts. If you do not live near a "recovery area", you could still find out ways to help and spend a day helping (organize a clothing/food/water drive, make personal care packages, etc)
The last idea is not really a field trip idea, but a good activity to go along with this topic. You could hold emergency drills, similar to a fire drill, but for other emergencies. See how quickly you can get everyone and all the necessary items into the vehicle in case of a rapid evacuation. See how quickly you can get everyone to a safe room in case of a sudden storm or tornado. These would be good to practice regularly, not just for this unit.
Hopefully, none of us will ever experience a major storm, but it is likely some of you have and most of us will. Preparedness is the best "defense" we have. Learning about the weather and the agencies available are a great way to familiarize ourselves with what can happen. That way, if it does happen, we at least know what we should expect.