What are some ways to motivate my Parkinson's 90-year old mother to do daily activities that she can do but would not?

This could be manipulation, but it could be that dementia is the issue. A significant number of people with Parkinson's develop dementia, and quite a few also develop dementia of the Alzheimer's type. I'd have a talk with her doctor and see if more tests can be done so you have a better idea of what she can really do and what is genuinely her disease(s).
You are right in that she needs some exercise, and activities of daily living can provide some of that. If she's manipulating, you may do accomplish more by backing off and not pushing. Once she feels she is in charge of what she does, it's possible she may do more. It's worth a try.
Take care of yourself, too. This is tough on the caregiver,
Carol

I was and am dealing with the same problems, tsolovie, with my mother who has PD. As recently as a year ago, I was pretty sure it was manipulation despite seeing signs of cognitive impairment, and now, as the disease is picking up even greater speed, I'm quite sure its the Parkinson's dementia. The best thing that I could and still do is to motivate my mother is to apply tough love by insisting that my mother do the things that I know she is still able to do, assist with those things that she may no longer be able to do alone, and do those things that are now beyond her abilities. Rationalizing and trying to reason with her (e.g., why it's important that she sets the table, walks, takes small bites, etc.) are becoming more and more frustrating and ineffective. Apathy is a huge part of some people's form of PD, like my mother's and, apparently, yours. I don't know about your mother, but mine was such a go-getter before PD claimed her. Trust your instinct when you wonder whether it's manipulation or cognitive impairment; my "gut" is and usually has been pretty on target, and I'm sure, since you're also around your mother a lot, yours is, too. Sending a big hug (and lots of patience!) your way.

She may be physically capable of performing the task at hand, however, the cognitivel issue becomes the problem. I have found that if I work with my husband and assist in just minor ways, when he is needing to "get moving", things progress more smoothly. He can shave and dress himself, but just being there to keep him on track seems to help tremendously. This takes a lot of time. We are the ones that must change. They are unable to go back to what they once were.The element of time seems to become lost to our loved ones. So whatever you can do to assist her and help her stay focused on the task might relieve some of the frustrations. Not only do they have Parkinson's, but WE HAVE PARKINSON'S. It becomes a household dilema. Take a deep breath and try to adapt to her changes, as there are many. Hopefully that will bring you some peace.

Well said, Carol.

Oops....Forgot to add...Carol has it absolutely right in her response to you. Sometimes, the harder you push, the more your mother may resist. I've found that to be very true in my situation with my mom.

I just read the statistic that 40% or more of Parkinson's patients develop dementia. Add to that that persons over 80 have a 50% chance of developing dementia, and you can see that Mother is at high risk for developing dementia. Does she have it now? Hmmm ... are there any other oddities in her behavior?

Perhaps backing off a little would be useful if she is manipulative (let her feel more control) and also is she has dementia.