When government buys back its bonds, it turns dollars over to bondholders. They, in turn, lend those dollars to companies that want to expand productive capacity. That helps the economy to grow, driving up the wages of future workers. The payoff of this plan is that high-wage workers will not feel much pain when it comes time to turn over taxes that are needed to support the elderly. But there are other ways to boost saving and investment.
Government spending on, say, early childhood education is also an act of saving and investment.
The act of reducing consumption today in order to expand output tomorrow fits the definition of investment. So does building airports and paying for basic research. At that level of generality, there is no answer. Economists attack the question one project at a time - with grunt work that tediously estimates benefits and costs. Consider a public project to create after-school mentoring and tutoring programs for young students.