Are you familar with Kims Game, The memory testing game where the participants as groups observe the items on display for a period of time, and then the items are covered up and then the group must list them.
within a time period.
I have used a variant of it using Nature items on a table with a tag telling what the item is. They can look at the display for 30 seconds. Then they turn around and the tags are removed. The judge then gives the group the tags and has them put them back where they should go. ( use an answer sheet for the judge ). Most correect wins. Use BOLD type written text not hand written text on the tags.
The items should be varied with most of them being something that a boy of this age level should know what it is, like a pine cone, for diffuiculty factor have two different pine trees cones displayed. Your items should fit your locale. They do not all have be flora and fauna I sometimes use different types of rocks, Quartz crystals seem to distract the boys so dont use them. they think they are diamonds. Turtle shells are really an attention getter also. I usually display no more than 25 items. Leaves wilt and turn brown, if you use them have extras, you can keep them iced in a cooler till needed. I always have a display of poisionous plant(s), but keep it in a plastic zip lock bag, but to make it interesting put a maple leaf in another bag. YOU MUST have some real whoppers for a few items, this will help establish a winner. Just for fun and to watch the boys think real hard I have sometimes had two of the same thing but of different size Big leaf, little leaf.
I use a grid on a sheet of plywood to display the items with each block numbered and then a scoring sheet for the judge. It is a timed event.
Depnding on your crowd have more than one station but they must be exact duplicates.
Arrange for prizes for the participants, games are no fun if you don't win a prize.
Have fun
JB