LabQuest 2 has a high-contrast mode for better viewing in sunlight. To activate high-contrast mode tap Home > Preferences > Light & Power, then check the option for "High Contrast". If necessary you can also increase the screen brightness when on battery power to improve readability of the screen.
For the original LabQuest, generally the screen is bright enough for fieldwork, even at 75% (default) battery level. For an even brighter screen, you can choose Power & Light from the Control Panel and increase the brightness to 100%.