Old Faithful Geyser, you can get here on snowmobiles or snowcoaches and a lodge to stay in for a memorial visit. Jim and I went many years ago and it was 30 below, painfully cold. The beauty out weighs the pain.
Mammoth Hot Springs, the north entrance to the park, always open to regular vehicle traffic. A quaint, historical, beautiful hotel to stay in. No TV, radio or room service, but you do have hot and cold running water in your room unless you choose to share a bathroom with folks down the hallway. Two suites do have cable now. No difference from when Jim and I visited 20 years ago. You get to bond with nature and wildlife instead of the outside world. I like that.