Nebraska's Ponca State Park
A few years ago I went on a trip to the north east part of Nebraska. When I arrived at Ponca State Park I was surprised at the lack of people. It was July and the place was not very busy at all.
Apparently at the time there was a lot of renovation going on around the park and that did not make it appealing to a lot of people. But, for the beautiful scenery and the activities available there, I was just shocked.
It is one of the most beautiful parks in Eastern Nebraska hands down. There are camping pads, cabins, and shelters there. You can purchase goods from the park retail gift shop or down the road a couple of miles in town.
The Park is also only 25 miles West of Sioux City, Iowa in case you get restless. I highly doubt you will though with the archery, horse riding, bird watching, golfing, sightseeing, star gazing, bicycling, ect.
The Missouri River is probably the highlight of this park. It's wide basin provides access to multiple areas for fishing and boating. There are docks around the park where you can gain access to this River with your boat and places for you to store your boat when it is not in the water. Be careful with the swimming in the area as the water is fast moving.
While you're out on the water with your spouse and you've left your kids behind, they can participate in one of the states most intricate outdoor educational programs at Ponca State Park. There are many programs that are integrated into the park daily activities.
If you aren't up for that, you could always go for a long walk. There are dozens upon dozens of trails around the park that you could get lost on for hours. I spent about 3 hours walking a trail that started low and ended up 150 feet higher at the top of the trail. It's a bit dangerous on some of the trails, as they are steep and require good walking shoes. Just make sure you let someone know that you are venturing out, if you are going out on your own.
Last but not least, do not fear... there are great shower facilities available to park goers all across the park and parking in front of them in case you have to park a distance away. http://nebraskastateparks.reserveamerica.com/campgroundDetails.do?contractCode=NE&parkId=230143