Okay, here's what I've learned from years of poking at 'The Simpsons: Tapped Out' whenever a new event drops: the game gives donuts for several legitimate, repeatable activities, and treating them like little rewards rather than something to grind for makes the whole thing more fun.
Level-up rewards and story/quest completions will sometimes hand you donuts, especially early on and during special arcs. Events are the richest source — seasonal events and limited-time content often include donuts as milestones or login rewards. Achievements can also leak a donut or two if you check them often. Also watch for official promo codes and social-media giveaways from the devs; I’ve snagged a few free donuts that way by following the game’s Twitter and in-game news.
Practical tips: do every event quest, log in daily to collect any calendar or prize wheel stuff, add friends so you get help on tasks (some neighbor interactions speed progress), and never fall for hacks or generators — those will get your account banned. I tend to hoard donuts for characters or buildings I really love rather than impulse buys, and that makes each free donut feel earned and satisfying.