Interval training of any kind would count: swimming, sprints, barbell exercises, et cetera. So would high-intensity exercises done for short periods, whether you repeat or not.
Twenty-rep barbell squats, for example, certainly count as high-intensity conditioning. So do burpees, and kettlebell swings, cleans, or snatches done for time (e.g. 5 minutes) or reps (e.g. 20 with a heavy bell or more with a lighter one).
Couplets, Triplets, and More
Another method of adding intensity is to switch exercises as you go. This allows you to continue to demand activity from your cardiovascular system without slowing down too much or failing due to local muscular fatigue. This might look like a set of pull-ups immediately followed by a set of air squats. In that case, since there are two exercises, it would be called a couplet. A triplet is alternating between three exercises, e.g. push-ups, sandbag cleans, and a sprint. (More than three exercises doesn't get a specific name.) Generally you repeat the set several times.
Crossfit is big on this kind of thing. For instance, the mainsite workout for last Friday was a four-exercise interval:
Four rounds for time of:
- 250 pound Deadlift, 8 reps
- 16 Burpees
- 15 foot Rope climb, 3 ascents
- Run 600 meters
If all the exercises involve one weight--say, a sandbag, barbell, kettlebell, or dumbbell--it's called a complex. People do complexes for the same reason as intervals, but generally the focus is a little more on strength and power.
My favorite barbell complex is clean, press several times, front squat several times, then repeat. With kettlebells, I like to swing, clean, and snatch (once each, repeating, or several times each and repeating). I also like to clean, press until I can't, push press until I can't, then jerk a few times before repeating on the other side. I like to integrate sandbag complexes with other work, such as sprints.