Leveling up in Hay Day is done by collecting experience points (XP), which are gained by buying things, harvesting crops and claiming items you create. One of the best ways to quickly gain XP is by filling out the truck delivery orders with the highest XP bounty, and making sure to throw out the orders with low bounties.
Another great way of leveling up quickly is harvesting wheat as often as you can, but only if you keep an eye on your crops. Make sure to harvest the wheat as soon as you can, as often as you can, to reach the optimal level of XP per hour.
However, as Hay Day forum posters pointed out, leveling up too quickly in Hay Day can be a trap. It’s easy to level initially, but an anxious Hay Day player may find themselves without the necessary coins to buy the machines needed to create the produce the game requires at certain levels. Instead, the best Hay Day leveling strategy is to allow the levels to come as they may, and build up a sizeable wealth in coins during the meantime.