Fallout 2 and Baldur's Gate 1, pick either original or Enhanced Edition, they're real good and are open world exploration games with main quest you can set aside to explore the world.
Very similar but less open worldy: Fallout 1 is timed and linear and Baldur's Gate 2 is more focused on character interactions in big city instead of exploring wilderness.
Very similar but less open worldy: Fallout 1 is timed and linear and Baldur's Gate 2 is more focused on character interactions in big city instead of exploring wilderness.