Rewards become unhealthy when they distort the behavior they are supposed to support. If the reward is too large, the user optimizes for the token. If it is too vague, the user ignores it.
A small exchange rate works because it preserves proportion. A short walk, a focus block, or a reset task earns enough screen time to matter, but not enough to erase the day's budget.
This keeps the incentive honest. The offline action improves the day on its own, and the screen reward is a secondary benefit rather than the only reason to move.
A good contract leaves the user with two records: the completed action and the limited permission to spend attention afterward.