There are four routes that come in with $49 one-way fares, but there are more than 50 at $59 or less. On the former, you can get round-trip tickets for just $97.96.
The sticker price might not be as wow-inducing as sales from Frontier or other budget airlines, but Southwest won't inflate your final price with loads of fees. In this sale, there's no fee for selecting your seat, and you get a checked bag and a carry-on at no additional charge.
So, get a flight and keep working your way through the US's best national parks or whatever weird travel goals you're working through. (If it's the former, maybe note the days when every US national park is free to help keep your journey as cheap as possible.)