Add an option to force the user to stop using Tux Paint
for a few seconds (10-30 sec), similar to the
'microbreak' feature of Workrave and other
RSI-preventing and "stretch-break" tools.
Make the feature, and the amount of time between
breaks, (and perhaps the length of the breaks)
configuration settings. Display a 5 second warning to
let the user know the break is imminent, then use the
standard "pop-up" dialog with a countdown timer. (Use
a clock, or maybe a pair of hands, though the latter
would really limit the break length to 10 seconds.)
Perhaps allow dismissal with Esc key?