What do you guys use as an opener for walk-around magic?
To get the reactions going to build a larger crowd, I used to get a group of 3 at least that looked like they were having a good time. Then, I would proceed to introduce myself and tell them to pick a card, all 3 remember it. Do the card change in the hands with all their hands covering the card getting them involved. That's one of the strongest tricks I do because it's so simple and direct, and it will literally get people screaming. Others will take note of that and start to come closer to see what's going on.
If some people come over after the trick is over, I would ask "Did you see that? Here, let me try it again, pick a card..." then go into another trick that you didn't do the first time. Get everyone involved, give everyone attention, and don't "break the threads" (Five Points in Magic - Tamariz). Usually a 2-3 Phase ACR, Card change in the hands, and Biddle (Anything simple and direct magic). This will keep everyone's attention and keep it at a nice pace.