Instead of another object they have to pack up eventually, why not give them an "event"?
Do they plan on having any formal dancing at the wedding--first dance as a couple, groom with his mom and then his MIL, and so on? Would they then like a gift certificate for dance lessons at a ballroom dance place? I know that some ballroom dance schools offer "wedding specials" of maybe two private lessons to teach the bridal couple the basics. That's a gift I would ask about first, frankly -- if for instance the groom is going to freak at the idea of ballroom lessons, even two hours of them, it would not work. It really is OK to ask about an unusual gift before you give it.
Or give them experiences like tickets to a play or concert you know they both want to see, or dinner plus a carriage ride (anyone can give them a dinner, but why not throw in something else to make it really special). Do special locations like Top of the Rock (top of 30 Rockefeller Center) have events? Do they really like doing something together like rock climbing (get a pass for indoor rock climbing facility) or cooking (certificate for a couple of cooking classes together)? Key it to their interests. They will get tons of household goods between the engagement and the wedding; give them memories instead.