I am going to put the pampas grass outside the pen because it is so razor sharp. I want it to cut predators, not the duckies. The bamboo is a great idea for outside too unless you have a really tall roof. Bamboo plants start out a deceiving height and often grow to 20 feet or even 60 feet in no time, depending on the variety.
I just planted 6 varigated liriope around the kiddie pool
I am also thinking of planting some day lillies right outside the pen. I don't know if they are safe to put in there with them.
And I am going to get some sod. for a strip. Maybe miscanthus would be another good one as an accent plant inside the pen.
Japanese forest grass is another pretty one
Or one of the blue ornamentals like blue festuca or blue oat grass
Or a red one.
I am going to fill them with white river rocks and pea gravel eventually.
See this link for many ideas or ornamental grasses.
http://directgardening.com/prodnav.asp?cat_search=Grs&header=Ornamental~Grass
Try a hardy banana musa basjoo that will grow in cold climates
Or a hardy palm that will also grow in cold states. Trachycarpus fortunei (windmill palm)
In general these gardens are called nature scapes... See this one...wow!