Moths - there are many that are very colorful and quite large. A flashlight or two can be used to attract them to a particular area. Constellations and Planets - use your parents as resources. It is amazing how many amateur astronomers you will find. Many stargazers have telescopes they may bring along to point out certain stellar features to the students.
Night smells such as night-blooming flowers or a surprised skunk.
A memorable end to a night field trip would be a campfire complete with toasted marshmallows. Be sure to check with local authorities to ensure you obtain all necessary permits. It would be wise to have the wood and tinder already set up so all you have to do is strike a match. Students or parents with scouting or woods skills can help with this. And what is a campfire without ghost stories.