Yellowstone is less than a two hour drive away and a beautiful National Park. Just take HWY 20 towards West Yellowstone and follow the signs, and you're there. This place is free for us because we have a year round pass. It's $25 for a pass to Yellowstone and Tetons, but just get a year round pass for $80 and visit all the National Parks within a two hour drive. (Tetons, Craters of the Moon) Yellowstone is exactly 100 miles away from the West Entrance and takes about 1 1/2 hours to drive there.
You can visit Old Faithful which is what the park is mostly known for, but I recommend the upper and lower falls:
Most of the roads are closed in the park from November to May, so go in the summer. Pack a lunch and make it a day trip. With a year round pass (free entrance into any National Park) and just a tank of gas, it's a great Saturday family activity.
