For peak holiday travel, fly from Houston to Miami for $292 roundtrip on American Airlines.
Searching AA.com, we found seats departing IAH on Tuesday, December 24, Christmas Eve, returning from MIA on Wednesday, January 1, New Year's Day. The outbound flight is nonstop, though the return requires a short connection.
This is a basic economy fare, but keep in mind that does include a carry on bag plus a personal item. You don't get to choose a seat and you board in the last group, but who cares? If the goal is to get from A to B as cheaply as possible on Christmas, this is how you do it.
As always, you'll find lower fares available for non-holiday travel.
For a look at more great deals, see our complete listings for Houston IAH.