SpaceX Space Launch Site
Brownsville, Texas
This is where SpaceX builds and launches its big Mars "Starship" rockets. Although it occupies an out-of-the-way spit of land, it's far more easily viewed by the casual roadside public than any tour-only NASA facility. The launch (and later wreckage) of Starship prototype SN8 in December 2020 was a highlight, but the retro-style stainless steel rockets are being built upright and outdoors -- at least for now -- so there's usually something to see. There are no tour busses and no pre-visit reservations; you just park your car by the roadside and gawk.
Get here quick before Elon Musk buys all the nearby land and blocks the view.