Running fartleks on a track is great. You sprint the straights, walk or jog the curves depending on how strong your cardio is for 4-8 laps total. This will make your heart uber strong. Also, where there is a track there is generally a football field. Another great sprinting workout is to sprint the 10 yd lines. So start at 0, then sprint to 10 yds. Then walk back to 0, then sprint to the 20yd line, then walk back to 0, then sprint to the 30yd line, then walk back to 0 and so on and so forth until you sprint the full 100 yds, and walk back. That's basically 10 sprints and this will get you breathing like a freight train. If you're uber into cardio, repeat from the 100 yd line until you've done a pyramid of sprints up to the 100 yd line and back again to the 0 yd line. That's 20 sprints. If you can do that without puking or getting nauseous, you are in excellent cardio shape. If you want to be in super uber cardio condition, try doing the full pyramid while lightly jogging back from each sprint instead of walking!
Sprints are proven to build a more powerful heart than long steady state cardio like long distance jogging and spare more muscle, or even build more muscle while losing bodyfat. The same cannot be said for long distance jogging or any other cardio activity for distance.
For those that can't run or don't like running, you can do sprints in a swimming pool which is another personal favorite of mine. Sprint 25 to 50meters depending on the length of the pool. Vary it between breast and freestyle, or butterfly if you know the technique. Then do a nice easy lap, then another sprint and repeat. This is very hard if you really sprint. I like breast stroke and freestyle the most for sprints in the water as they both tax your cardio in a different way do to the breathing patterns used.
I'm sure you can do sprint style workouts on rowing machines and stair steppers too.