I found Mammoth Spring State Park to be an excellent place to stop and enjoy the day. The centerpiece of the park is a huge spring, which gushes millions of gallons of water from underground sources. The park provides an overlook platform, as well as walking trails and walking bridges all the way around the spring.
In the summer, you can even cruise on paddleboats in the lake. The park has many features normal to state parks, such as picnic areas and a playground. But this park also includes a railroad depot musuem, a caboose, a dam and powerhouse tour, as well as a national fish hatchery next door.
In years past, people believed the spring waters to have healing powers. The town of Mammoth Spring was, therefore, popular with those wanting to bathe in the spring waters. The depot was the major way in and out of the town. Today you can tour it, and, if you're lucky and like trains, even have one pass by while you are there.
Also, there is a dam and powerhouse you can walk on and tour. This was an early source of electricity for the region.
The Mammoth Spring National Fish Hatchery has a great aquarium building, as well as huge concrete fish tanks, where you can see thousands of mainly trout being raised for release around the region. It's free, and can be walked to or driven to from the state park.
In addition, there is an Arkansas Welcome Center on site at the state park.
It's actually possible to spend the balance of the day here, and have a great time doing it.
Motels in the town of Mammoth Spring are limited, but there are more along U.S. 63, either in Arkansas or Missouri. And you are quite literally on the state line when you are at the park.
I give the park great marks for a good leisurely time.