INSTANT HAPPINESS EXISTS. And it’ll only set you back around $35 a go. Pools of sprinkles, confetti domes, a digital cheese cave—you wouldn’t be blamed for thinking the Museum of Ice Cream, the Museum of Pizza and other similarly frothy exhibits popping up in Los Angeles, San Francisco and New York were attractions aimed at children. But you’ll find plenty of grown-ups sans kids filing into these new-school museums. What’s the enticement? They’re all-immersive, begging-to-be-photographed fun houses. Most are pop-up exhibits but a few have set down roots, filling a seemingly insatiable need to feed Instagram feeds. Here,...