Going on a 5-6 hour flight. Game recommendation?

If you prefer some story-heavy games, you can try Eastward. It has gorgeous art style and controls and UI are a great fit for handheld devices such as Deck and Switch. It takes ~15% battery a hour and can be completed in 10 to 15 hours.

If you're fine with longer games which will take 45-70 hours total, try some 2D JRPGs like Trails in the Sky FC (which also has nice touch input in menus!), Trails from Zero and Chained Echoes. All tree aren't battery-heavy at all.