If you're feeling bored or burned out without your cardio or strength program, it's time to try a ladder. The idea is simple: you go up and/or down in reps, weight, distance, or increments of time. So, doing three reps of push-ups, then six, then nine. Or dead-lifting a 100-pound barbell, then 125-pound, then 150-pound. Or you could sprint 100m, 200m, then 300m, or jump rope continuously for 30, 60, then 90 seconds. In all of those examples, you could start at the harder end of the ladder, and progress it to be easier and easier, or you could go up and then back down. The point is that you're always switching things up, which keeps you mentally engaged.

 Total-body movements, like a squat-to-overhead-press with dumbbells, are especially ladder-friendly, because they tend to be done in short bursts, and you want to get as much as you can out of your time. Alternatively, you can cycle between complementary movements, such as kettlebell front squats and pull-ups, since they tax two different parts of your body, Salvador says.
Like any new workout, you'll want to start conservatively to see where you stand. Don't construct a ladder that has you doing 300 cumulative reps, or reaching the end of a weight progression that has you going for a one-rep max. The ladder should feel tough, but the point is to finish the workout, not go crazy in the first few minutes and bonk by the end. Consistency is the goal.

This is also a buddy-friendly workout. Even if you're not at the same fitness level, you can adjust weights to your own abilities. Then you can still follow the same ladder structure and have a compatriot to help you push through those last, more painful rounds.
Finally, "retest the same ladder from time to time to see how your strength and overall fitness are improving," says Salvador.  Because you're hitting such specific numbers during any kind of ladder, tracking progress will be easy. 
Here, three ready made workouts to try:
Squat Strength Descending LadderThe squat is one of the most complete movements you can do, as it not only strengthens the glutes and legs but also activates the core and lats. Use a kettlebell front squat for this workout, it will help you go lower, and it requires more muscles to stabilize. Hold the kettlebell upside down (handles down, hands on the bell) at your chest. Squat down as low as you can, keeping your chest up, and shoulders back.
The workout: 12 reps, 10 reps, 8 reps, 6 reps, 4 reps, then 2 reps; all as heavy as possible. You're aiming to increase the kettlebell weight as the ladder reps go down. Start with something light for the set of 12, then go a little heavier for 10, heavier still for 8, and continuing to progress until you hit a weight for 2 reps that feels challenging but doable.
Conditioning Rep LadderWhen you're looking to up your aerobic capacity, you want less technical movements where you can go hard for a shorter amount of time — hence the box jumps, sit-ups, and push-ups here.
The workout: 3 reps of each exercise, then 6 reps, 9 reps, 12 reps, etc, continuing for 8 minutes without stopping. The key here is that, while these three moves are simple, the rate of work should tax your lungs and your mental fortitude. 

Sprint Ladder Repeats

A sprint ladder for time will reveal how fast you really are. While most of us can hold a dead sprint for 10 seconds, pushing for an all-out run for 30 full seconds is a tall order. You'll want to head to a track for this workout, or get on a treadmill.

The workout: 10-second sprint, then rest one minute; 20-second sprint, rest one minute; then a 30-second sprint. Take three minutes rest, and repeat three to five times. Your goal is to push yourself to go a little bit further for each progressive round.