I'd say that if the couple has a registry, please stick to it. I didn't understand that at all until I was married in June this year. My husband and I received several gifts from well-wishers that were not on our registry. While we appreciated the gestures, we're now stuck with so much crystal glasses/vases/bowls that we don't know what to do with them. They're actually locked up in our library on top of the bookshelves for fear of our cats breaking something.
If you really feel that you don't want to go the registry route, one of the best suggestions I can offer is photo albums and picture frames (if you know the couple's tastes). We have so many pictures from our wedding, but we're still lacking in frames.
