I think a sunken trampoline has to be my favourite family garden feature. It& a simple enough idea but solves so many headaches. It& safer, less intrusive and popular with everyone. I have sunken a couple myself.
Good idea for multiple reasons: equisetum (horsetail, snake grass, puzzlegrass) (grow zone grows so quickly and make even the newest newbie of a gardener feel accomplished and happy. equisetum can get out invasive and a container will control it.