Medicare HMO
If you don’t want Medi-gap insurance you can enroll your elderly parent in a Medicare HMO.  He/she has to keep Medicare A & B but they use the doctors and hospitals in the HMO.  One nice thing about an HMO is that it covers preventative care that is not covered by Medicare.  An HMO also covers hearing aides (if you can your LO to wear one), dental care, and eye glasses. You pay monthly premiums to the HMO.

Be aware that if your LO wants to get out of the HMO they have to apply for Medi-Gap insurance which could be more difficult if they develop a new health issue.  They may not get guaranteed access to a policy.
Doctors and hospitals can go in and out of an HMO which may leave you with fewer options.  When you inquire about joining an HMO find out how many doctors have left.  Ask about switching doctors if your doctor leaves or if you are dissatisfied with his/her service.
Since the elderly use so many prescriptions, be sure they can fill your current medications.  Find out how often they update their formulary.  How and who determine what is medically necessary and how can you appeal that decision?  What happens when your LO needs emergency care while traveling beyond the area covered by the HMO?