I've seen several things done and appreciated.
A nice family portrait.
A framed picture of them at their wedding, along with a print out of the lyrics to "their" song.
A picture of them at their wedding that was sent to one of those places that does a painting of it. WOW! It was spectacular.
A new garden installed...complete with flowers, fountain and bench to sit on to enjoy it.
Nice rocking chairs for the front or back porch.
A bench made with all the children and grandchildren's names and hand prints (even the adult hand prints)
A ring and lapel pin made with the kids and grandkids birthstones.
A DVD made of pictures of the family accompanied by music of their era.