I think one idea would be restriction of state funded pensions and benefits. The onus of looking after old people should be with their children and grandchildren, not the state.How many of you have kids? They are exhausting. If you don't already have one, you cannot truly comprehend how tiring raising children is.
And unlike everything else, development and rising living standards make raising children harder, not easier.
Most couples, Chinese couples included, still have at least 1 kid. The problem is, after people see how tiring raising one kid is, half stop having kids, and the other half have at most 1 more. For modern parents, subjective marginal returns from having additional children turn very negative very quickly. That's not enough - you need every couple to have at least 2 kids, and some to have 3, to keep the population stable.
The marginal returns from additional children are so negative, that I don't think there's any reasonable inducement any government can offer that can get people to have 2-3 kids/couple on a societal level in a modern developed country. Raising kids in our modern world is just too much work, especially compared to everything else which has been made easier and more comfortable by technology.
It would restore the financial incentive to have children that has existed forever until recently.
It would be difficult to do in an electoral democracy as old people are the biggest voting bloc, but would be easy for China to implement.