Running is one of best activities you can do. It doesn't require special equipment (except some quality shoes) and you can do it anywhere. Best of all, you burn serious calories, especially if you add hills and sprints. A 145-lb person can burn 300 (at 5.2 mph) in 30 minutes. The downside is, it takes lots of practice and you should watch your knees and ankles for any discomfort or pain.
Outdoors or indoors, cycling gives some great cardio. Using all the power in your legs, you'll increase endurance while burning lots of calories, anywhere from 250-500 in 30 minutes, depending on how fast you go and how high your resistance is.