A gym full of wide-eyed kids clapped along as Jack Johnson put on a three-song mini-concert for Mililani ‘Ike Elementary School on Wednesday.

It was an acoustic, good-natured preview of what’s in store for Hawaii fans on Aug. 1, when Johnson and his band will play a headlining show at the Waikiki Shell. Tickets for Hawaii residents go on sale at 10 a.m. Saturday at ticketmaster.com and the Blaisdell Box Office only. Online sales will be open only to those with a Hawaii-based credit card billing address.

“I grew up in Hawaii and know how disappointing it was to get shut out of shows,” Johnson said after the performance. “That’s something we learned years ago at the Kokua Festivals.”

This time around, Johnson and band will be looking for “an acoustic feel,” he said, with guests, if any, yet to be decided on.

“I’m excited to play at the Shell for the first time and not have to think about hosting a festival,” he said.

Smiling widely at the kids during the show, posing for photos with student council leaders and teachers afterward and waving to youthful fans on the playground afterward, Johnson was tanned and relaxed, wearing slippers, shorts and a T-shirt as he sang, standing with his guitar. Afterward, he talked about his upcoming show and environmental efforts, including a bottle cap challenge that won the Mililani kids this show.

All of Johnson’s concerts in Hawaii support the Kokua Hawaii Foundation, he said. The Kokua Foundation, in turn, funds a school recycling program and Plastic Free Hawaii initiative.

At the school, he sang, “We’ve got 3 Rs to talk about today,” to a reggae-tinged melody played on his acoustic guitar.

“Reduce, re-use, recycle … 3, it’s a magic number.”