The most appropriate gift for a wedding present, outside of what they register for, would be kitchen, bath or bedroom accessories. A marriage is a melding of two households, typically single, and you are helping them to make their new home together, so there is usually quite a bit of things from those three categories they do not have yet.
I think it’s always really safe to follow the couple’s wishes. Most couples nowadays have a bridal or wedding registry so you’ll know exactly what they want and need. If the couple is not registered anywhere then they will usually have an alternative gifting option listed on the wedding invitation or the wedding website. For instance, my friend had instructions on their wedding website that they preferred monetary donations so they could pay for their honeymoon.