Trail running....with hills/elevation if you have it in your area.
Trails work your entire core....stepping around and over rocks, roots, etc. Lose focus and you'll eat it.
If you have an area with hills, then you have intervals already built into your route. Proper form moving up and down hills will strengthen core, knees, joints, etc.
This makes road running feel very boring, not to mention treadmills and cardio machines.