If you're too hot to run outside, and you refuse to exercise indoors on a treadmill, I don't blame you. That doesn't mean you need to give up on your workouts, though. Slip on your bathing suit, find the nearest pool, and start treading.
It's essentially like running, but you're floating in water. You'll feel resistance from all angles, so it's an amazing thigh-, butt-, and core-toning workout. If you use both your legs and arms to stay afloat you'll also tone your upper body. Complete an interval workout while staying cool by alternating between treading water for two minutes, and resting for one minute. While you rest, you can float on your back or stomach, or stretch on the side of the pool. Repeat for a total of 10 times and you'll have completed a 30-minute cardio workout without even breaking a sweat.