1) Running 5 sets of 3mins at moderate-high intensity (70+% of MHR), with 1min breaks in between.
2) Rowing 10 sets of 30secs at very high intensity (90%+ of MHR), with 1min easy rowing in between.
3) Multiple "rounds" of bag punching, immediately follow by 10-12reps of DB/KB swings/cleans/snatchs, rest 30secs to 1mins (aka Mike Mahler's HOC)
4) 8 sets of (inserting your favourite cardio exercise here) as fast/hard as you can for 20secs, rest for 10secs. (aka Tabata)
The above comes from my personal experience and they worked well for my conditioning. The combinations and possibilities are endless.
Hope it helps,