You can buy these fabulous things called "tree blocks", but personally I think taking the kids out in the woods or stopping on the side of the road when you see a tree fallen down is a much better idea than buying them. You can then cut them to size and hey presto a wonderful resource to enhance play and a wonderful addition to your block play resources.
Carefully, Creative... Crash!!!
Enhancing imaginative Play
Supplement with stones, shells and pine cones to create more opportunities, and use outside.
"You can discover more about a person in an hour of play than a year of conversation." Plato