In the US, there are many things a person has the right to do. Regarding these things, what actions are legal for person A to do but which would be illegal for person B to do even if formally authorized by A? Only one comes to mind:
(How) Can Person A grant immunity to Person B for ending Person A's life?
Are there others? If specifics are needed, suppose the state is Oregon.
Legal Representation: You have the right to represent yourself in a legal proceeding, but you cannot assign that right to anyone that you choose, only to certain approved individuals.
Vote: You can vote, and you cannot assign that right to another.
Jail Sentence: You can (indeed must) serve a jail sentence or be executed for a capital crime yourself, but you cannot allow another person to do that on your behalf.
Military Draft: When we had the draft (most recently), you could not authorize another person to serve in your stead.
Marriage: You can marry your neighbor's sister (if she is not related to you), but you cannot transfer that right to your neighbor (who is the brother of the intended bride).
These are different things from committing suicide, but it's not clear exactly what concept of exclusive sovereignty you're getting at.
Similar to @user6726 item about legal representation,
Building Trades: Person A can do various carpentry, electrical and plumbing work on the house they own and reside in, but are restricted in their choice of person B to do it for them.
Healthcare: Same with healthcare, I am guessing. Person A can slice off their own [skin tag] or sew up their wound, or take medicines, but outside of certain emergency situations (e.g. anaphylaxis), are restricted in their choice of B for medical procedures.
