Have they not got a wedding list? Most couples these days set one up and many do it online so just ask for the details. If they haven't, you asking may give them the idea to set one up anyway.

I agree with the cash option but would also add something more to this - a smaller gift they can open. Something like a nice bottle of wine/whiskey/whisky/port/whatever they like to drink and some nice glasses to drink it out of.

You could also go for paying for some aspect of their wedding like buying the cake, booking the photographer or booking them a car to take them to and from the venue and for a drive afterwards thereby saving them some money.