Fly nonstop from Detroit to Miami for $145 round-trip on American in regular economy. A few summer travel dates are available.
For example, I was able to find seats for a one-week trip over 4th of July, departing Detroit DTW on Tuesday, July 3, returning from Miami MIA on Wednesday, July 11.
You'll find other dates available for travel from May through September, though most availability will be found in late August/September. A few scattered peak summer dates can be found for mid-week travel. Look for travel to Florida Mondays through Wednesdays and travel from Florida Tuesdays through Fridays.
There are slightly lower basic economy fares for $133 round-trip, but I reccomend shelling out the extra $12 for this fare in regular economy so you can bring a normal size carry-on bag plus one personal item. The lower basic economy fare only includes a small personal item and other restrictions apply.
Need a room in Miami? Here's what's available for the above dates on Hotelwatchdog.