Each reached summit rewards you with Will, which is used to upgrade four stats. Strength lets you push faster, Vitality increases stamina, Recovery increases the recharge rate, and Fortitude reduces stamina consumption. Each stat can be upgraded several times, with every level costing more Will, but the bonuses always increase by one unit and there are no sudden price spikes.
Putting a few points into Strength is a good idea because it shortens each trip considerably, reducing runs that initially take around 10 minutes to just 2 or even 1 minute. You can also spend Will on new types of builders and mountains. Depending on your preference, you can switch to a heavier boulder and tougher mountain that generates greater rewards, or choose an easier setup that lets you reach summits faster. Everything has proficiency levels, which unlock additional perks the more you use each option.
After some time and several summits, you will earn your first Spark, which is used for another set of upgrades. The amount of Will and Sparks you generate from each summit is clearly displayed, allowing you to calculate how long it will take to reach your next upgrade. Sparks are much rarer, however, and it takes several summits to earn even one.
The main difference between Will and Spark upgrades is that Spark upgrades are permanent and remain with you after rebirth, which is the game's term for prestige. Rebirth becomes available after reaching a summit 25 times and resets all of your Will upgrades and boulder purchases, while keeping proficiency levels. If you want to spend Sparks on upgrades, you have to rebirth first because they cannot be purchased during an active run.
Spark upgrades are divided into several categories that improve different aspects of the game and unlock new features. For example, the Body category contains several upgrades focused on stamina, while Encourage improves the efficiency of the Encourage icon that randomly appears on screen. Clicking it cuts several minutes from the time needed to push the boulder. A similar icon, Lightning, appears whenever you reach a summit, and clicking it grants additional Will.
One particularly useful category is Fate, which unlocks several quality of life features. These include offline efficiency, which allows you to earn more while the game is closed, and auto purchase, which automatically buys selected upgrades, or eventually all of them once the feature has been upgraded enough.
However, the more useful and powerful upgrades are locked behind various requirements. Some require you to purchase another upgrade first, reach a certain number of summits, or complete a certain number of rebirths. Others require all three.
If you have the patience, everything is eventually obtainable. The game is clearly designed to be left running for weeks or even months, but that is also the point. It is a pure idle experience that does not demand your attention every few minutes. You can check in occasionally, make your upgrades, trigger a rebirth when appropriate, and then leave Sisyphus to continue his eternal climb.