Christmas 2014, my wife and I spent camping on an otherwise uninhabited car-free island in the Pacific Ocean. On Christmas Day, we hiked all day in superb weather in a spectacular landscape, less than 100 km from the Los Angeles area, and we did not see a single soul. It was the best day of my life. And we travelled there from Toronto, Ontario, most of the way by train
Montañon Peak, Santa Cruz Island, Channel Islands National Park, Christmas Day, 2014. Probably the best day in my life.
Although the train is not a typical mode of transportation in North America, is it quite suitable for an outdoor vacation in combination with car rental. You're not limited much in the amount or type of stuff to take, you get to experience the distance and see the country, and you can get closer to most national parks than by air. Compared to driving, it saves time, because the train continues day and night.
We took a train from Toronto, Ontario to Salinas, California, then a bus to Monterey. From there we had a one-way car rental to Los Angeles, camping at the Big Sur and at Channel Islands National Park on the way (possibly the most underrated national park in the USA; see above). Then we took a train from Los Angeles to El Paso, from where we had a one-way car rental to San Antonio, while visiting White Sands National Monument, Carlsbad Caverns National Park, and Big Bend National Park (we could have camped there, but it was winter and we decided to take a cabin, which was for the best because it turned out a serious winter storm was coming). From San Antonio we took the train back to the Great Lakes area.
It was a very memorable vacation and we did not enter an aeroplane once.