Rock Spiral Garden. This is the first one I've seen that I like because it closes on itself instead of leaving an opening leading to nowhere. Now, an open one could lead to another point of interest, but I have yet to find one that does.
[ Idea: Designing an herb spiral ] Click through to check out this herb spiral a few years after planting. Be sure to also visit the link that shows what it looks like at the beginning. http://www.hipchickdigs.com/2012/05/herb-spiral-update/
20 Tree House Design Ideas to Fill Backyards with Fun