My wife and I have been going down to Coeur d'alene for the past 2 years and we really enjoy it. We went to Silverwood the first year and its a pretty nice park - we paid $80 each for the 2 park pass - so you can get all the rides you want and then after go and enjoy the water park.
We camp just outside of Coeur d'alene and last year we rented a 18' Sea Ray for reasonable price from the campground. It was $250 for 4 hours.
There's actually quite a bit to do in the area - Hang out at Lake Coeur d'alene (nice beach) and they've got a nice bike path system. If you golf, they've definitely have some reasonable priced courses, and then they've got a beauty in the Coeur d'alene Golf Club. They've got a floating green that if you land your first shot on the green, they give you a certificate. Never played it as it's pricey, but that's what I've heard.
For breakfast, check out this place:
http://www.michaelds.com/
So good.
So there's actually quite a bit to do down there, and the wife and I are trying to decide if we're going to go again because it's a little different camping with a 4 month old baby.
You mentioned that you're doing a road trip - where else are you going? The first year we went down, we stayed in Coeur d'alene for 3 days, drove over to Seattle for Independence Day, and then drove north and stayed in Victoria for a week. Then back to Calgary after that. It was great trip, and well worth the time and money.