Me Please! :)
Not sure what data you are storing...Maybe you could think of an algorithmn such as select model id & the count of number of days that model has actually logged on. Then you take the model with the top count & display & then set status to 'Displayed' or whatever, then the next day it will get the next model with top count where status <> 'Displayed' and then you'll be able to run through the entire set of mods in this way. Also you could limit the selection based on whether a mod hasn't logged in for a certain period of time, etc....