Just because they are all adults doesn't mean they wouldn't enjoy a stocking, especially a surprise one.
Maybe base it on the poem:
Something you want
Something you need
Something to eat and
Something to read
Wants could be gift cards to favourite coffee shops, puzzles to keep them busy, lovely cosy gloves or scarves, nice small jewellery....
Needs could be nice toiletries, especially travel sizes. A travel mug for the M2B could be good (so she can actually get a hot drink once baby arrives). Funny socks, or really nice soft socks.
Eats - fruit and sweets for Christmas, and maybe things like spice mixes, nice coffee grounds, different flavoured teas, special biscuits etc...as long as there is something to eat immediately, you can have things to use in the weeks ahead too.
Read - a nostalgic annual, a book of jokes, a good novel that you CAN put down or short stories (thinking especially of M2B in that one), I had a book of Sudokus at my elbow every time I sat down to give a feed! Anything about the history of your or their local area?
My Dad still does stockings containing fruit (red apple, green apple, orange, satsuma, banana and (kiwi, nectarine or pear)), sweets, and a book - for every person under his roof on Christmas Eve. If there are kids there, their own parents sort those, but Dad still does all his own kids even though they are all well grown now!!