Core conditioning is fundamental to overall health and besides the mountain climbers exercise, burpees are really challenging as well. If you are not familiar with the burpee, you're in for a treat! It is tough and the video I found demonstrating it below even combines the burpee with mountain climbers. Overall, it is a great exercise utilizing only your body weight to build up your core as well as your upper body (arms - throwing in the push-up and/or pull-up.)
Burpees are also a good option to throw into an interval routine. They are an excellent whole body workout and can be done almost anywhere. If you are traveling, pressed for time, or just don't have access to a gym, you can get in a good workout just doing this exercise.
Burpee exercise demonstration: