Can I give a discount when the cart reaches a certain amount, like "spend $100, get 10% off"?

Yes. Create an automatic In-cart order discount with a Minimum purchase amount. Tiered discounts by cart value ("spend $100 get 10%, spend $200 get 15%") are not a built-in feature yet. As a workaround, create a separate discount for each threshold.

Learn more: In-cart order discount - settings/targeting · Combining different discount types