Hi OP, I’m so terribly terribly sorry you are going through this - can’t really imagine what it must be like.
I have no direct experience but my mum lost my brother as a baby, and although I was too young to remember him, I know all about him. When I was older I realised that although mum has some keepsakes the most important thing for mum was being able to talk about her son especially on significant days like his birthday.
I would be a little wary about planting trees and the like if there is any possibility that you may move at some point or your children will move away from this special place and not be able to take the object with them.
I think it would be much better to do things that are portable. Maybe a photobook for each of you - maybe take a photo on the same day each month so you can see how much your son changes/grows in the time you have with him. A hand or footprint artwork.
Also be aware about how your children would feel if they inadvertently damaged or lost one of the special things you’ve got. So maybe not jewellery or the like
So forgive me if this is a bit insensitive but I wouldn’t worry too much about trying to make special memories, just try to enjoy the time you have together as a family without putting more stress on yourselves. You will always remember your son and your older DC will too because you will talk about him even years in the future