Then ask! You won't be left guessing and hoping for the best. You'll be told some ideas at least, and you can pick and choose from there. And, when in doubt, anything homemade and personal/personalised is a good way to go. It shows you put a lot of thought and effort into it. For instance, I assume they'll have a photographer? You could pay for and customise a few frames for pictures that they can put into the frames. If they like coffee or tea or wine, you could have cups or glasses made with their names and wedding date on them, or anything else on them you think they'd like. There's something called a ring bowl that you could make with their initials and dates, and it'd be something to ring bearer uses to carry the rings if they're having a ring bearer. There's also, if you can, getting pictures of them and making custom coasters, which is a little silly sounding but it's got that personal touch. I've also seen some customized pillowcases that match the bride and groom (such as "his" and "hers" and "Mr." and "Mrs." pillowcases).
If you're going for more practical items, there's barware, beach towels for any potential beach holidays, gift certificates to stores or restaurants they might like (which could make for a nice, romantic dinner for them), luggage for the honeymoon, tea kettle with nice teacups, baking ware (especially if anyone is a baker or cook), flatware, fine china, casual china, a nice set of knives, extra picture frames for all the photos they'll likely have from their wedding and honeymoon, possibly a wedding album or a nice album for the photos to go into, extra towels in general, extra bedding in general, and anything storage related. If you know where they're going to honeymoon, you could arrange a couples massage if it's somewhere that has a spa and pay for the massage yourself or pay for one more local. You can buy items that correspond with their hobbies and leisure activities (such as tents if they're campers).