Trains and buses.
If you really love adventure, it's right downtown at the Greyhound station. The passengers are generally politer than anything on a plane these days. It's the best way to see the countryside, and if you ride one down to Mexico you can hook up with the best network of buses I've yet to see, with bustling terminals like mini-airports.
If you don't want to leave the US, you can still go international with the half-hidden routes of China buses or Mexico buses. These two usually stop near bustling ethnic emporia that are interesting in their own right. If you are White, you may have to convince people you really want to ride, but that will be a heartwarming part of your future travel tales.
Maybe you say, "Cool, RD, but I can't help it, I am just too damn fancy to ride on a bus," well I gotcha brother. Book Amtrak and get a big-ass fancy sleeper room. The conductor is your guard and servant, and you have dining privileges vs the Irish down in Coach. There will be inexplicable delays and/or shortages announced in brutal fashion over the intercom by people fired from subway systems for poor enunciation, but it all just washes over you as you lounge on your own private toilet seat.