I like Watson's idea for holiday ideas.
Gift cards are impersonal but welcome since they can get what they need/want.
Since you mentioned you don't know they couples very well, I'd pass and have passed, on a wall hanging. Would they have a place for it? Fit into their décor? ...and you'd be putting so much work into the w/h.
Mug rugs with cute coffee mugs and a poem what the rug are for could be a fun novelty gift.
You may get some great ideas when you can see their registry. Good luck.