What is the best cardio workout for toning the abs?

I am looking for a cardio workout that will tone and stregthen my abdominal muscles. I am looking for something that I can do at home. I am not a fan of going to the gym. I already walk 2 miles at least 3 times a week. I am looking for something a little more intense. Can anyone recommend any exercises? I do have an ab-lounge, is that all I need to use? Any suggestions will be appreciated. I am not looking to lose weight. I just want to tone up.