Embrace the picnic
The food you find in Yellowstone won’t be anything special, so the least you can do is eat that not-special food while looking out at something spectacular, rather than sitting in a drab cafeteria (though I’ll allow that the Wilderness Chili is pretty good).
The move in Yellowstone -- at least for breakfast and lunch -- is to go to one of the general stores you’ll find next to the visitor centers. A single company supplies all 12 stores within the park, so the inventory is more or less the same wherever you go.
You’ve got the most options at the largest stores: Old Faithful Basin and Canyon Village, where you can curate pretty much whatever genre of picnic you want: Survivalist (granola, Beefaroni, jerky an’ stuff); Survivalist: Low Carb (meat-and-cheese sandwiches, by which I mean a sandwich that is literally meat sandwiched by two pieces of cheese); Bougie (coconut water, wine in containers that look like they should be for coconut water, Greek yogurt); Pre-Assembled (grab-and-go salads, sandwiches similar); and Family/Walmart (everything on offer from PepsiCo, General Mills, and Unilever). You’ll obviously cover more ground if you eat on the go, but to really do the thing right, you should just take your bounty to one of the many scenic picnic sites around the park and enjoy it there.
Definitely try the ice cream
There is one exception to everything in the park being just OK: the ice cream. If you’re visiting in the summertime, you’ll notice there is ice cream at just about every store or kiosk, and it’s good. Try the huckleberry, a flavor which an assertive branding campaign has installed around the park in the form of everything from coffee to pancake mix. It’s very good.
For a national park in which so many people get around by driving, there is a truly stunning volume and diversity of alcohol available for purchase. At pretty much any of the general stores, you’ll find local beers with quirky names you probably haven’t seen before: Otter Water, Moose Drool, Bitch Creek, Highlander Bison Tale Pale Ale, Old Faithful Ale, Twisted Tea (huckleberry flavor), Teton Amber, and Family Vacation. Drink responsibly, of course.