Nobody has said jump rope yet? What is wrong with you people.
Jump rope.
Running is still the best time/workout thing around. If it gets boring fill a backpack with sand and run with it.
There are also a ton of suicide variations:
Sprint 200 yards/do 20 pushups/Run 200/ do 20 pushups/repeat
Swim 1 lap/do 20 pushups/repeat
Skate from the red line to the blue line back to the red line then to the centerline then back to the red line then to the far red line. Repeat going the other way.
swimming>>>>>running