You could hire a local hall for the morning so that most of the talking can be done out of the sun. On-site demonstrations and field tours can be arranged for the afternoon. Speakers Field days are for showing not talking. There should be a maximum of exhibits and discussion, and a minimum of speaches. Speakers should have well-prepared, simple charts, graphs and specimens to show the audience.
People have a very short attention span when listening to talk unsupported by other methods.
The people attending a field day all come from a range of different backgrounds and experiences. Don't assume everyone has the same level of awareness or understanding of the issues you are discussing. It is a good idea to make sure everyone is up to speed on what will be looked at on the day before starting. This can be done by orientating participants to the site and providing a summary of what will be covered, including advice on how to get the most out of the day. Guest speakers Restrict the number of guest speakers from outside the district to one, as what they say is often not relevant to the local scene.