Honestly, the easiest thing is to go down the public library, and get a book of vintage games! Stuff for "around the campfire" will have a campy theme, and back in the 50s and 60s everyone had outdoor parties anyway. No bouncy houses, no hired ponies or reptile experts, definitely no Chuck E. Cheese venues for 30 kids! I did this for our kids and the books have the "rules" and supply lists, which are pretty easy. So I commend you for having a small manageable party at home! (Sack races using old pillow cases are good, so are potato races and bean & spoon races. They aren't about the fastest and biggest kids winning anyway - they're about patience and balance, so it's a pretty even playing field. But only do that if competition is a healthy thing for this mixed group, and not if it's going to get too pushy!
We did a lot of outside treasure hunts too - you can get plenty of plastic eggs on sale now anyway, but they are always at party stores. We put small candies, small dinosaur-type toys (like those foam things you put in water), plastic bugs, and so on in the eggs and then hid them. You can sort them by color, giving each child a color to search for so everyone gets something. If you have more kids than colors, put a sticker (either something related to the theme, or just a colored dot sticker from the office supply store) and then give each kid a plastic or paper bag with his sticker color on it. (That helps the little kids who don't read so well). Big kids can help little kids - make the goal to find all the eggs, not to be first. What's in the eggs replaces the goody bags - no added expense. You can use nature-themed goodies too like little magnifiers for examining bugs and other finds.
You could also do a scavenger hunt for things like a pine cone, a maple leaf (draw an outline so they know what's what), a 2-pronged twig, 3 rocks of different sizes, some moss, a seed or pod of some sort, anything else that's native to your area. You can be general like "something green" or "something brown" and you can also include general categories like "something man-made" (they'll find a bottle cap, a penny or an old toy, anything). You can also put stuff out there ahead of time if you need to.
You can also make some sort of tent out of clotheslines (or ropes & bungee cords) and old sheets, serve cake in there. Better yet, provide the materials and a little adult supervision, and let them create their own - they'll have a blast and that will be the activity! Use a tarp for sitting on if the ground is damp. Take photos and email a few pics to every kid's parents.
If you have a grill or fire pit, make marshmallows and/or s'mores. The kids can collect their own sticks or you can buy long sticks in most craft or outdoor stores.