Perhaps instead of paying out the dividend payment as usual, you can make the user receive tickets based on how many index fund shares they own, and you can fun

Perhaps instead of paying out the dividend payment as usual, you can make the user receive tickets based on how many index fund shares they own, and you can fund a lottery using the dividend payments. The user can sell their index fund at anytime, so it would be the same as keeping their money in the savings account except they have better odds of beating inflation than a savings account. If you limit the savings options to tried and trusted savings vehicles - savings account, total bond market, total US market and total international - perhaps you can instill investment knowledge in the users on top of the good savings practices.