2 approaches (a) Same goals for everyone
(b) Set individual goals
I think (a) would highlight weaker participants and wouldn't have the desired effect. (b) is difficult to accomplish as everyone is different and it would be impossible to manually assign individual goals to everyone. An approach for (b) would have the first day / week as a basis to beat for the second day / week and then the second day / week a basis for the third and so on.
These following solutions also take into consideration the previous research done on self-efficacy and encouraging physical activity.
Solution 1Ideally I would like to create a system with 7 different cities within the one world each city representing a day in the week. On the first day they are walking or doing extra activities to build up New York. Then on Tuesday they are building up Paris. When walking on Monday their aim is just to beat the previous Mondays step count to receive extra credit.They won't be able to build up New York till next Monday again so it gives renewed motivation and it is easy to distinguish which days a user is putting in more effort. For example Mary could be viewing her world and New York is twice the size of Paris obviously she doesn't put in much effort on Tuesdays.
Solution 2 (The more feasible and applicable solution)A person will only have to beat the step count of the previous day and will be building up the same city every day. To display their previous attempts they will be able to request a graph of their previous days or weeks which will chart all the step counts per day against each other.
Solution 2 will be the first prototype because of time constraints but if feasible Solution 1 will be the revolution of Solution 2. Both comply with the requirements but Solution 1 entires of a vast scale in comparison to Solution 2.