We always fly to Orlando (MCO) for 49/59 each way. The nonstop airlines from DTW are Spirit, Northwest, Airtran, and Southwest. I usually book air 6 or so months in advance.
I am waiting for prices to come down for our August trip. Right now they are pretty high. If you have time to check everyday it will help. The prices for this past Oct on Airtran and Spirit were both 49 each way, but it didn't last for very long. Also, these prices are not the prices that they email me that there is a deal going on. For some reason those deals are never as good as the ones I find myself.