Haha, I like how SushieBoy just cuts to the chase.
As opposed to, say, a telephone with Mario's face plastered on it, I generally enjoy merchandise that looks like it would belong in the Mushroom Kingdom. Also, I don't like it when something is easily reproducible--say, a button or a plastic sign. It just seems less special.
Bearing that in mind, I would love the opportunity to purchase a music box from SMB3. That would be cool to me. Also, for some reason I've always pictured them kind of big, so they'd have to be about a cubic foot in size... I'm writing this on my to-do list, in case I'm ever in a situation where I have access to woodworking tools...