Is this a good idea for a wedding gift?

So this will be first wedding that Ive ever been too and Im wondering on what a good wedding gift would be. Im a groomsman, I know about the registry and people that I ve asked is just to get something off it. Im pretty good friends with the bride and groom, and kind of feel like I should get something more. They are going to go to vegas for their honeymoon and I was thinking of getting them tickets to a show while they are there (The Blue Man Group). I dont think they know much about them but heard its a good show, costs a little to go to, so I dont think its something they would normally splurge on. Do you think this would be a good idea, as well as picking up something on their registry?