Plain old air mattress is a ball. 100 foot line to get in the flat wake. Hold the rope with both hands under the pillow. Let one of the smaller kids sit on your back.
Or how about a parasail with 1200' line. I built an electric level wind spool to play out the line. It quick connects to the back of my 21' condor.
Kids are grown now and I don't want to kill off any grandkids so it just sits in the bsmt.
In the 70s a 4' circle of plywood was popular. Not tied, more like a weird (slow) ski.