Coins are used to upgrade both your crops and your weapon, with each category having its own separate stall containing several upgrades. The crop stall starts with four upgrades, including more crops per respawn, a chance for crops to explode, and a chance for larger crops to spawn. The ultimate objective is to grow a crop so large that it eventually blocks out the sun.
The weapon stall has six upgrades, with options such as increased damage, faster fire rate, more projectiles, and a few other improvements. To make it easier to spot upgrades you can currently afford, they turn yellow and slightly pulsate. Although every upgrade is useful, you want to prioritize the chance of spawning additional crop types, as these crops give you more coins per kill.
Respawning is generally free, so you can do it as often as you want. However, there is a cost if you try to respawn while crops are still on the field. In that case, you have to pay the combined value of all remaining crops. This cost is not too significant at the beginning, but once you unlock more valuable crops, it can become extremely expensive. To save money, it is always best to shoot everything before respawning.
Eventually, you will max out all the upgrades in all stalls. This unlocks the option to upgrade the stall itself, adding more shelves with new upgrades. These can include things like a coin magnet, an expensive auto collect upgrade so you no longer have to run around picking up coins, ways to move valuable crops, and additional upgrades for ones you have already maxed out.
You can also purchase the quest stall, which gives you a random quest in exchange for a nice chunk of money. The stall has its own upgrades that can increase the payout, reduce the cooldown period, meaning the number of respawns required before another quest becomes available, and add a couple of other improvements. Quests usually involve hunting down a fast crop, killing a specific type of crop, or completing some other small challenge.
However, the ultimate goal of covering the sun is not possible during the first stage. Once you max out every upgrade and have nothing left to buy, you unlock the next area: an ice level where you shoot different types of crops with a new weapon. All of your upgrades are reset, but you also get access to a few new ones, such as turrets that you can place anywhere on the field. Overall, the upgrades in this area are more expensive, making progression slower and giving the stage a different pace.
Once you complete the second area, you move on to the third and final area, the oasis. This is where you continue grinding, pushing through the remaining upgrades and working your way toward the endgame. It’s a nice distraction for a few hours, and comes with a fair price tag.