I think get them one leg standing with the eyes closed - on or off a pillow depending on their ability. Best place is infront of a kitchen sink so they can hover the hands in the sink and if they fall they'll catch themselves easily.
Thus the test and treatement is how long they can maintain the position without falling - the patient is testing themselves each time by timing to see how long they can do it (someone else can time for them).
Good progressions are minitramps (rebounders), wobble boards, and doing on tip toe (my favourite because it's horrendously difficult). - eyes closed and eyes open. working towards being able to do it on tiptoe on a pillow eyes closed. Depending on the age of the patient you can say they should be doing it for 30 sec or a minute or 2 minutes.