Another thing to try could be to rank people in realtime instead of the one-off submission approach. I do this in https://spaceword.org (create tight crosswords using 21 letters), and I think it's quite motivating to see how you compare to others as you improve your solution. On the other hand, its a bit more taxing on the server, and then you also could not show the optimal solution.
layer8|1 month ago
michaelmior|1 month ago
oliwary|1 month ago