Or: guess confidence intervals. 95% might not be as useful as 50%; test yourself not only on how often you are under or over, but make sure that 50% (or %5) of the time it is outside the range you guessed.

If you try to guess things that you're really sure about, this forces you to quantify how sure you are about that, and makes those guesses no more or less useful than those that you are much less sure about.