Craters of the Moon is an intersting National Monument that has several miles of trails, some beautiful caves and is a great place for your family to explore.
There are several short trail loops and out and back trails. It is a good idea to get maps from the visitor's center at the entrance and also to bring some good headlamps so that you can explore the caves.
Toward the back of the park there are longer trails. These trails are less frequented than the shorter trails near the front of the park, and make a great place to get in a long trail run.
You should be warned that if you go on a hot day, because of the black lava fields, it gets REALLY hot and will be uncomfortable for hiking. The best temparatures can be found in early fall. There is also a good chance you will see some wildlife while you are wandering around.