It all started with a donut. On March 22, right after the first day of spring, Krispy Kreme offered a free glazed donut to customers who showed their COVID-19 vaccination card. Since then, the chain has given away more than 1.5 million donuts. On Friday, National Donut Day, all customers can get a free donut. If you’re vaccinated, you can have two. As the first day of summer approaches, vaccination incentives have become a growth industry. They include everything from million-dollar state lottery giveaways to full-ride, four-year scholarships at public universities, from hunting and fishing...