United has an advertised sale basically matching the latest Southwest sale for travel through October. American, Delta, and Alaska are matching along many routes as well. Fares start at $97 round-trip.
For most routes, travel is valid on Tuesdays, Wednesdays, and Saturdays, with a 14-day advance purchase required. I've also found Sunday, May 27, available on most routes. The United sale is advertised for travel through June 20, but like the Southwest sale, most of these fares are also available from August 21 through October 31.
Travel to Nevada and Florida is valid Sundays through Wednesdays; Travel from Nevada and Florida is valid Tuesdays through Fridays. You may find travel on May 24, 25, 28; August 31, and September 3, to be blacked out. A Saturday night stay may be required depending on the route. There also may be a 30-day max stay. Click on the fare details for more info.
Tickets must be booked by May 10, 2018.
A sample of fares include:
There are many more routes and these fares all work in reverse as well. Most prices are for basic economy, which means United and American will only include one small personal item (small backpack, large purse, etc.) that will fit underneath the seat in front of you. Larger carry-on bags are not allowed. Checked bag fees start at $25 each way. If you would like to bring a larger carry-on bag, consider the higher regular economy price.
Delta still includes a normal carry-on allowance plus one personal item in their basic economy fares. Alaska does not sell basic economy yet, but they plan to do so this fall.
Above image by Vitalii Matokha via Shutterstock.