If you can’t find anything on the registry, a gift card for the amount you can afford to the place at which the couple is registered is always a good idea.
When I got married, the best unexpected (i.e. not on the registry) gifts were those that our friends had given us because they wished they had gotten one earlier or that someone saw and thought of us. (Silicone baking mats, random board games, nice Tupperware containers…) What memories do you have with the recipient(s)? It’s always nice to put a bit of personal thought into a gift, particularly if you’re having trouble coming up with something.