That question is so big that it can never be fully or truly answered. The options are great, and choosing between them is a defining experience for any van-dweller. The driveway or yard of a friend is ideal, but the truth of who your friends really are comes to the fore when it comes time to make such offers. Soon, though, the time will come to transcend the urban, and the possibilities open up even further. There are showers at truckstops if one is between gyms, but all that is ever really necessary to shower is a gallon of water and a hand towel.
Rest areas work well, and they're on the map.
There is a small gravel parking area just below the airport vortex in Sedona. It's about twenty bumpy miles to the north rim of the grand canyon. You can park right on the rim with nothing but the sound of the Little Colorado Spilling into the Colorado. You can't rent that, or buy it even. There is no way to purchase the intensity of the omnipresent immediacy of choice brought into one's life by van-dwelling.