How about a living table for your outdoor dining, complete with a rivulet down the middle?
This striking table caught my eye as I scrolled through my Pinterest feed the other day. It looks to be constructed of COR-TEN steel and would be a permanent structure wherever it were built.
It would certainly be a dramatic focal point of any garden, although in this size it would need a fairly large garden to fit into nicely without being overwhelming. I do think the idea could be downsize for smaller gardens though.
Source: flickr.com via Douglas on Pinterest