I agree with the others about the fishing. You cast it for him, but let him have the opportunity to catch something. Watching you catch fish while his line just sits there won't be any fun. Once he's reeled in, you can start showing him how to cast.
Other activities: photography - a cheap digital camera can get them very excited about the outdoors. A scavenger hunt. Get a book on insects and go bug hunting and identifying. A tree identification book.
I also agree with letting them help with setup, take down, and all that's in between. Kids under 9 are eager to help and want to do a good job. Let them sweep out the trailer, let them help plan the meals, and so on. They'll love it and will remember that for years.
1975 American Clipper RV with Dodge 360 (photo in profile)
1998 American Clipper Fold n Roll Folding Trailer
Both born in Morgan Hill, CA to Irv Perch (Daddy of the Aristocrat trailers)