If had lots of stumps a could do this, otherwise just three or four and space put a little can be used for balancing or table and chairs
Nest for kids -- what a great reading spot! Grape Vines could be interwoven into the nest.
Willow forts! They grow super fast! You can start a new tree from just a small cutting. Leave it in water for a month or two to get long roots, then plant it outside! Very easy to shape.
How To Build A Living Playhouse That Helps Kids To Understand Nature http://theownerbuildernetwork.co/1gjw Here’s a fun and educational way to divert kids from the indoors to the great outdoors… help them to build a living playhouse!
Use different height stumps/logs to create a climbing structure. Incorporate this into the bow of the boat somehow?
Pallet Projects - DIY Outdoor TeePee for a Kids Playground or the Backyard - Do it Yourself Outdoor Woodworking Tutorial via 1001 Pallets
Backyard Chalkboard--less mess and the rain washes it away! such a great idea!
Balancing beams that double as benches Low enough to be safe whilst still allowing children some form of risk