I always just put money in the card.
Traditional wedding gifts are often unnecessary for a lot of couples, who have already lived together for some time. Most, however, go on an (often expensive) honeymoon and could benefit from the cash for that.
We did ask for money for our wedding, but received quite a few gifts from people that maybe didn't feel comfortable with that. All gifts were really appreciated. There were quite a few more traditional gifts. One person did buy us a share bag of sweets. That gift really stood out because it was so different from the others, and it was lovely just to sit down and share it once the wedding rush was over.
But, honestly, your friends will appreciate any gift given. We loved every gift because they were generous presents.
The only one that we struggled with, personally, was a 'token' money gift for a very small amount. Not because it was a small amount, but because it was given to us by a family that would spend all of their time bragging about how much money they had. We'd rather they'd have given us nothing. It's not about the gift, in that case, but the message behind it.
Some people did give us nothing, and that was fine as well. It didn't leave us with any negative feeling.