Also known as the Terrace Outfield in left and the Sun/Moon Decks in right, the Lower Outfield is where you'll want to be if you're looking to bring home a long ball as a souvenir.
But be prepared with plenty of sunscreen on day games as these seats see some of the most sun of any area at GABP (especially in the right field sections). There is some relief to be found in left field however, as the last few rows (J and higher) of Sections 101-105 are covered by the Outfield Bleacher deck located just above.
Sit in Section 101 to be right by the Reds relievers as they warm up, or opt for Section 140 if you want to check out the visiting team bullpen. Section 140 also puts you close to the Bow Tie bar and Riverwalk, making it a good option for socializing during the game.