After selecting a tool, tap the Return key to quickly highlight the first text edit field in the Options bar. For example, selecting the Brush tool and then tapping the enter key will select the Opacity option.
When working with panels, dialog boxes, and the Options bar, clicking on the icon or text next to the desired text edit field will highlight it (which can be easier than clicking and swiping within the text edit field to select the value).
Once a text edit field is active, tap the Tab key to move between fields. For example, with the Brush tool selected, in the Options bar, tap the Tab key to move between Opacity, Flow, and Smoothing. Shift + Tab moves in the opposite direction.
In addition, once the contents of a text edit field is highlighted, the up/down arrow keys increase/decrease the selected value by one unit. Add the Shift key to increase/decrease the value by 10 units.