For my 2 sons who turn 12 and 16 one month apart.  We decided to have a giant slip and slide.  This can easily be constructed from a roll of platic sheeting.  I purchased the clear 100' roll.  Roll it out on an incline if possible.  Place hay bales covered with plastic on the end.  At the top you will need a plastic tarp or two underneath as well as some sort of padding like carpet remenants or a tumbling mat. The kids can either wear a swimsuit or cover their clothes with trash bags w/holes for head, and arms.  We purchased tearless baby shampoo and slathered it on their bellies. Wet the slide down and let em have at it.  They really enjoyed it.