Keeping it Simple and Varied
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If you plan to get all the way to Chattanooga on Day 1 with a four year old, then you aren't going to have much time for major stops. And even so, you're not going to make it in 8 hours. Plan on 10 and also plan on making a quick stop every few hours to let your son get some exercise and fresh air.
In the Sevierville area, the obvious place to check out are Great Smoky Mountain National Park (Be sure to get the age appropriate Junior Ranger pamphlet for your son and turn it in to a Ranger when completed.), Dollywood in Pigeon Forge, Chimney Rock Park in Asheville and the Hickory Ridge Homestead in Boone.
AZBuck