The kids put up there objects they find outside while walking or playing, or seasonal crafts. Think of it as the extension to your nature notebook... it might display a caterpillar or ant farm, a shed snake skin, birds nest or some robin's eggshells. Maybe you'll be braver than me, and place the wasps nest on the table rather than on the ground outside ;)

Ours is the top of a bookshelf in the kitchen, and also houses some of our religious things-- sort of a family altar slash nature table. There used to be a cross there but it disappeared, and I need to replace it, and the Bible occasionally rests there for a while ;) The kids have placed a bunch of rocks on there, and the two baggies hold some European Hornets ready to mail to collector friends! There are some shells and an empty frame that occasionally holds a picture made by the kids. The flowers are silk, I liked the colours and hope to get some real leaves soon!
Be that as it may, our table is just starting. If you like, take a look at Katherine's table for a great idea!