Lots of good ideas here, but at least in the case of many in the US there are some intrusions of reality. Most of the things suggested can probably be done, but in some places it may take some prior planning.

There is much variation from one jurisdiction to another, but I have encountered the requirement, in several places, that any "below ground burial" must be in a standard vault. The "vault" usually is a reinforced concrete box that the casket of your choice sits inside. Some caskets, usually only very expensive ones, are "self-vaulting" with, the seller assures us, sufficient strength and resistance to deterioration to provide the function of the concrete box.

In at least one instance, the bereaved were informed that even burying a can full of decedent's ashes required the standard vault - and of course the full size standard burial plot in a "registered" cemetery.

Legal restrictions on "scattering ashes" exist in some, if not all, Federal Parks, Wildlife Sanctuaries, etc. Of course, if someone objects you might just offer to pick them all up ... (Enforcement is variable.) There are, of course, numerous State and Local ordinances, ad infinitum.

In the US, an apparently widespread and thriving business has been found wherein certain mortuaries have been collecting organs without the consent of deceased and/or family. There have been a few very unfortunate cases in which diseased organs from these sources have been used as transplants. There seem to be few functional¹ legal barriers; but in any case where one does not want bits and pieces donated², perhaps having survivors informed that they should include in the contract for services a clause prohibiting any such harvesting would give some assurance.

¹ It's of course illegal; but extremely profitable and apparently quite easy to get away with.(?).

² A person afflicted with cancer, hepatitis, AIDS or other autoimmune disease, and possibly numerous other peculiarities certainly wouldn't want something passed on(?).

The bottom line is it takes prior planning, documentation (often in legal form), and broadcasting of the plan to sufficient people likely to have a part in your final arrangements - before any plan remotely out of the ordinary is likely to work as you might hope. It may not take a lot of extra effort, but when it's time it's too late.

John