All we did was head outside with paper and crayons. Then we gathered leaves and other natural items and placed a thin paper over them (we used cheap copy paper. I think old paper bags, or used craft paper would be great as well.) and the little ones rubbed over it with a crayon. Then the textures magically appeared. We also ran around and rubbed over walls, sidewalks...
One thing we found out was, even though we may see a certain object, a sidewalk for example, every day, we can still discover hidden details about it.
I never knew how full of texture everything is.