The Achenbach Trail can be the most epic trail in the North Unit of Theodore Roosevelt National Park when done as a loop and combined with the Buckhorn Trail.
The first part of the trail ascends up the side of the canyon but is not too steep. After about 3/4 of a mile you start to get some great views of the valley below.
The "cannonballs" are easily spotted at their namesake pullout in the North Unit of the park. A short trail goes around the strange reddish-brown spherical objects.