My friends and I make annual trips to Idaho every first week of August, and of course at least a day of Silverwood is on the itinerary every year, but I've started skipping it and finding something else to do while everyone else goes. Like rage2 said, go during the weekday. And if you don't have to be there all day, go during the evening. There's even a reduced rate if you go after 6PM or something. There will be almost nobody there and you'll fly through the line ups and pretty much get the same number of rides as going during the day. You gotta try out the Aftershock at least once. First time I went on it, some kid collapsed to the ground crying after it ended cause he was so terrified haha (and it IS pretty fucking scary)
Oh, and the ice cream cones are great value. They're big enough for two adults or an adult and two children hahaha
IMO, 6 days is a lot for Coeur d'Alene, unless you're planning on spreading it out between there/Spokane Valley/Spokane. Definitely rent a boat and spend a day out on the lake swimming and tubing if you can, especially on a +40C day. In Idaho, you
don't need a boating license to rent a boat. They'll walk you through how to operate the boat before they send you on your way, even if you know how. Wayne Gretzky owns a place on the lake haha
If you're stopping in Sandpoint, The Pie Hut is worth having a meal at and of course some pie.
If you're looking for booze, make sure you check out Total Wine in Spokane Valley if you don't know about it already. I don't drink and even I enjoy going through all the aisles looking at all the shit they sell.