Midway park is highly overrated. It has a small beach along with a small amusement park. Unless you have a child that is within the age vacinity of 7 I doubt your family will enjoy the park. Many of the rides are geared towards smaller children and the park needs to be updated.
Unique Suggestions: If you must go to midway make sure to bring a swimsuit so you can at least relax on the beach. Also, if you're going with a group of people bring a volleyball; they have nets in the sand.
Fun Alternatives: There aren't any other amusement parks around the lake, but there are some nicer beaches and I would suggest renting a sea-doo and taking advantage of the beautiful lake right at your fingertips.
The town of Jamestown has practically nothing interesting in it unless your a fan of Lucielle Ball. She was born there and there are several tributes to that. Anything good about jamestown isn't in the town it's the great sites around this area. Avoid the city itself if it can even be called a city, it's more like a big hole. Alot of the places I mentioned are not right in Jamestown and may be closer to other cites Jamestown just happens to be where I went from to see them.