don’t tie the rope to a wall, I did as a small child, it hurt a lot, and my mum put me on the kitchen table and proceeded to sew 3 stitches to an anasthetic free forehead
most impressive toy I’ve seen was at Centre Parcs, a 4ft wide 30ish foot long ‘rubber’ mat was slung, suspended, about 3 ft above ground between three trellises (ie 3 fixed points with two runs between them) sag was still about 1 ft above ground, person A lay on mat bewteen trellis 1 and 2 and person B jumped up and down between trellises 2 and 3, by hitting the subsequent wave correctly you could get person A about 3ft up in the air above the mat
if kids are small, tyre, upright embedded in ground, plank roped to top of tyre, might work as seesaw (*all liability disclaimed)
take care if you live on a hill, we found that you can put your mates in a (tractor) tyre and roll them down the hill
if you wrap old foam cushions in waterproof plastic/gaffer taped up that acts as a crash mat