Product-based challenges aren't about getting a customer to repeat the same purchase — they're about breaking the habit of ordering the same thing every visit and cross-educating guests on the rest of your menu.
A lot of regulars settle into one set item and never explore anything else. A product challenge gives them a reason to try something different without feeling like they're being upsold. It's fine, and often the point, to require the purchase happen across different visits rather than in a single order.
A "menu explorer" challenge — requiring a purchase from each menu category — is a straightforward version of this. Decide up front whether that has to happen across multiple visits or can be completed in one visit; it changes both the difficulty and the pacing of the challenge.
Dessert is a specific example worth calling out: it's an add-on that frequently gets missed, so a challenge built around trying dessert (or requiring purchases across multiple categories that include it) is a simple way to lift average ticket while also diversifying what a guest has tried.