Turnout in American elections is low in part because voting is logistically difficult for people who work hourly jobs, don't have flexible schedules, and can't afford to wait in line for two hours on a Tuesday. The problem is not motivational.
Making Election Day a federal holiday (call it Civic Duty Day) is the simplest possible intervention. It doesn't change who's eligible to vote. It doesn't change how votes are counted. It removes a structural barrier for the people most likely to be turned away by the current system.
Countries with higher turnout have made voting easier. This is how you make voting easier.