Suggestions:
1. Show progress towards some fixed number of questions so you can see your score at the end so you can see how well-calibrated you are and compare with friends.
2. Add other things than weight, so people can get calibrated for other things too. Time. Probability. Money. Risk of death. Deaths (in war or from disease, e.g.). Stuff like that!
3. Have a speed mode where you have to learn to do these calculations really fast, so that you can incorporate this awareness into your day to day life and awareness of things. 4. Have a slow mode where you are trying to get everything right, so that you learn how to proceed when you really want to be sure.
5. Have a mode where nothing is within 100 or 1000 of anything else, so that you're purely focused on scale awareness and so that your ability to estimate on a 10-100 scale doesn't matter so much.