The railyard, the daily bonus, and doing neighbor visits are the best ways to get free donuts when you are starting out. If you want to donut farm in the future you should collect items with bonuses and make sure they are all out of storage. The Beach Hideaways are the best, followed by the Jet Engine Bikes. The Springfield Heights Mansions, Business Centers, Condos, and Private Islands together contribute 153% xp, which will help you to level up and earn bonus donuts once you pass Level 60.