Got an Answer?
The same as you'd give newlyweds in their 20s with no kids.
Marriage is marriage.
At least their partnership has lasted.
Assuming you are going to a party..... buy them a nice gift. Just because they have been together years doesn’t mean they don’t need things or things replaced.
$100 voucher for dinner somewhere nice, couples massages.
I know people are happy for others to simply enjoy the celebration with them.
$50 in a card. At this point, given what you've said about them, it's going to be a token gift, anyway. They don't need anything.