For example, I found seats departing Denver DEN on Friday, September 7, returning from Baltimore BWI on Wednesday, September 12.
Other dates are also available for travel from late August through mid-November. You may find similar fares on Spirit, though do keep in mind that Southwest allows for two free checked bags, while Spirit only includes a small personal item. Southwest is hands down the better deal in this case.
Need a room in Baltimore? Here's a look at what's available for the above dates on Hotelwatchdog.