I didn't think we needed a store dedicated entirely to M&M's. I still don't.
But if you're traveling with a kid - or someone that acts like one - there's a good chance you'll end up at the M&M's World store on the Strip. My niece wanted to go - so of course, we did. It's 4 floors of madness. When it's crowded (it almost always inexplicably is), it's pretty intense in there... breathe deep and get through it. The giant wall of every shade of M&M is pretty and will entertain you for a few minutes. Non-chocolate items are predictably silly and useless but it's likely you leave with something, so suck it up and pick a color already.
You could really skip this shop, which has become almost a Vegas attraction of it's own. But if you're walking by it - and it's central location practically guarantees that at some point in your trip, you WILL be - pop in and get it over with.