You could do something with high/low to bet if a water temp is going to be hotter or colder than a starting temp if you wanted to make it a game of chance. You could do something as basic as rock/paper/scissors, or tic tac toe. Pick n games that are that simple, simplify your sodoku to 1/9th the size, do multivariate / a/b testing to see how the CTR is for each game and let the traffic/users decide.

FreeRice is also nice because the domain is short =) although it looks like free/clean water/h2o variations are squatted (as far as instantdomainsearch.com knows) - good luck!