Just a 40 min drive from Spokane, Coeur d'Alene is a beautiful lakeside town. You could visit the resort and marina, golf the floating green or cliff jump at Tubb's Hill.
Explore Riverfront Park, feed the trash goat, ride the Looff Carousel, see the Pavilion built for World Fair, grab dinner at Anthony's and visit Jonah's favorite Spokane shop, Boo Radley's.
A 5hr drive from Diamond Lake you can explore one the Pacific Northwest's beautiful national parks. Drive "Going to the Sun" Road, paddle board at Lake McDonald or hike to Avalanche Lake.