Also, do you want to buy something for their home that they might think of you when they use it? Or a present that can only be used once? (eg balloon flight, bottle of champagne). And what is your budget?
I have just bought my friends a beautful bird feeding station for their garden. I got it at a local garden centre but they are available on line also. It only cost 25 pounds and is made of black wrought iron. they were thrilled with it. It may not be to everyones taste but it is an unusual gift and these days when most couples have already set up home and have the usual irons and toasters it is something different.