johnt-
Pleased to meet you and welcome to our world.
I have also thought a bit about meaningful measurements. Some ideas-
1) When to take a measurement? The same time each day doesn't work because our state fluctuates. There are a few times, however, such as when we first wake up. There is also the time when we are at our best in our cycle, as well as at our worst.
2) It is also important to record just how much of our day is spent in either an "off" or "on" state since those are indicators of quality of life.
3) Since we all have keyboards, using it as a measurement tool is good. Timing oneself in typing a standardized sentence measures manual dexterity. Again, the time-in-cycle needs to be consistent.
4) Length of stride is a good "macro" measurement. I lay out a tape 100' long and hit on the zero with the first step and count them to the other end. Then do a little math to get a "two-step" stride.
5) I use a timed balance on one leg test. I stand sideways in an open door frame, fix my eyes on a point on the frame, and raise on foot off the ground. I count chimpanzees until I am forced to put the foot back down. I don't accept the test unless the count is above a certain value such as 10 or 20. Depending on the circumstances, I might take the best of three or I might average them. The important thing is to be consistent. Then I do it with the other leg.

This last one is pretty handy since it requires nothing in the way of equipment. It also gives a snapshot of several things at once. Leg muscle strength. Sense of balance. Cognitive (counting). It can be pretty precise, especially if you repeat it several times and average.