ask the player to give a 90% confidence interval

This could be important. Some card games teach calibration, such as Bridge and Spades. (Although it's not quite the same, because after you guess how many tricks you'll take, you have some control over it - if you were underconfident, you can throw tricks away, if you were overconfident you can take unusually large risks.) But they just ask for a single number, and later you see how close you were but you can't look back and see how confident you were. If you give a confidence interval, it's much easier to see whether you're well-calibrated.