You're looking for a unique, fun way to make your BBQ for grown-ups awesome. Here's one suggestion: MOJITO WATERMELON POPSICLES. Yes, this is a real thing. No, I didn't make it up but I wish I could claim this idea.
While you're hear, I might as well brag a little. I visited the latest restaurant in the Pok Pok family, Sen Yai Noodes, on Friday. If Pok Pok chef Andy Ricker isn't the closest thing to The Food Messiah, I don't know who is. I mean, really, everything I've ever eaten in his restaurants is so delicious I could see myself bathing in it. You've tried the fish sauce wings, yeah? If not... Well, I'm just plain sorry for you. Here's a recipe you can TRY to get an IDEA of what they're going for. But, if I were you, I'd just let them take care of it. It's worth it.