The height/circumference ratio is an excellent suggestion in terms of psychology and health, something that interests me from the parenting perspective, but it neatly side-steps the intended physics lesson. :-)
I've already considered the water displacement method but it is not practical for a single individual to both stand on a raft AND measure the displacement. You also have accuracy problems because the relatively large surface area of a bath tub makes for small displacement magnitudes giving a...
I have teenaged daughters, and like many of their peers, they're obsessed with their body weight. Unfortunately, since I have access to accurate scales at my gym, I've never bothered buying a bathroom scale at home, and this is now becoming a source of friction.
Rather than simply surrender...