how to get straight hair wavy

I've been doing fine straightening the wavy part of my hair (the back) but since it's been getting so hot and I get no a/c in my bathroom I'd rather not be drying my hair when it's 90 degrees outside. The front of my hair (bangs and sides) are 95% straight and the back is wavy. I guess I either need a really good straightening product so the back doesn't get frizzy or some way to make the straight part wavy so I can go natural. Is this possible?