Make relief rub prints using crayon - lay a piece of paper on shapes and patterns and run the side of a crayon on it. Press leaves in books 'tween wax paper. you can buy solar paper where you put something on it and lay it in the sun and a shadow picture emerges (amazon.com might have it, if not a science site online). If the youngest ones are boys, bring Hot Wheels and a scraper to make a little track on the ground for them. If near water, make boats out of milk or cream cartons.(get ideas online for the sails.) A memorable craft to highlight the reunion? Make a launcher for things like rockets (out of soda litre bottles), marshmallow guns, simple slingshots, or a solar oven out of foil, duct tape and a pizza box (ideas online) then they can do s'mores or lil English muffin pizzas in them.
Bring Mentos and a bottle of diet Pepsi and watch the volcano! There are so many science minded fun ideas you could do- I would definitely think along those lines. Kids will be busy wanting to explore and the campground can offer much outdoor fun. Take some bubbles, kites, flashlights for night fun,(a printout from online of campfire spooky stories) , a pocket guide on birds, trees, plants, or butterflies, a bug collecting jar with a wire mesh lid and magnifying glass. When my kids were very little we would collect rocks and dip them in water to see the "new" colors emerging on them. My girl would weave nests /baskets out of dry grass and twigs, (its called wattle weaving) make fairy houses, forts, burn leaves with a magnifying glass in the sun (with supervision), make mud pies (take some metal pie tins), look for moss on the north side of trees and explain the science behind it, teach them how to skip rocks on the water, make a fishing pole from a tree branch (bring line, hook and bait), or build a simple bird trap with a box, string and stick. Bring a raft with oars. Visors: glue pearl strings, glitter,fabric bugs ,flowers and butterfIies to fabric visors. If you collect shells, rocks, moss, bark, feathers, sea glass, etc.....you can make a collection either there or when you get home. You get a shirt box and on the lid glue a piece of paper that has some kind of graphic with "my collection" on it (might find them online?)...get a light colored construction paper the same size as the box. Using a ruler,mark up a large grid pattern on the construction sheet and glue it inside to the bottom of the bottom box. (if it were me I would do this before I go so all the kids have to do is color/glue the graphic for the front lid, and glue in their specimens. From experience I have learned to do as much of the prep for them as possible in many more in depth crafts or they burn out before the craft is done.) The kids can glue a piece of their collection into each grid area- they can write underneath it what it is. It could even be a leaf collection (and if you bring a little 'pocket guide to trees' book , you will be able to correctly label each tree leaf.) We did one of these for a seashell collection and it is one of my kids favorite collections. (we used the internet when we got home to look them up and ID them). We also did a shirt box to hold a variety of things when we did a Colonial Day for school. You could make a graphic: " My (name) Family Reunion Treasure Box" and all the crafts they do, and collected things can go in the box .Include post cards or campground maps and maybe a few small wrapped treats. Sometimes the dollar stores have small plastic trowels and magnifying lenses.... that could be cute in there. If you need something larger than a shirt box- use a brown grocery sack (preferable with handles) that you can get free from the store. Put the graphic over the store logo. Use a big sheet of colored construction behind it to hide more if you need to. Take some 'insta' plaser of paris and make forms out of animal tracks (and even kids footprints.)