Like its punk-pop heroes, Green Day delivers the goods with such seeming ease that it feels as if they aren’t even trying. But a sweet, catchy tune like “Nice Guys Finish Last” doesn’t appear on anyone else’s album and after two decades of punk rock, Green Day manages to find a way to keep itself energized with the creative fires burning. All successful bands are threatened by an overexposure that’s out of their control. The best a band can do is diversify and try new approaches. Which Green Day does within its own punk parameters. “Hitchin’ a Ride” adds a trudge. “Good Riddance (Time of Your Life)” goes acoustic with strings and became a huge hit. “King for a Day” adds horns and a quick-stepping beat. “Last Ride In” serves as a south-of-the-border surf instrumental. “Redundant” is about as close as punks like to come to a ballad. These mild diversions widen the trio’s scope, but the heart of the band can still be heard in the straightforward propulsion of “The Grouch” and “Scattered.” Green Day’s appeal is so simple, it makes one question how can it be so?