Many ways, and it depends what you are good at. The easiest way to make a future return safely. Is to invest money monthly into the S&P 500 index fund. The S&P 500 is like 90% of American business overall, so you’re esentially “betting” American will be worth more in the future, which is highly likely.
Here’s the maths: the cumulative average year of year for the S&P 500 (in the long term 10-20 years) is 10.4% annually. So, $1000 a month, every month, for 22 years. And you are worth $980,000, but only invested $240,000. After 9 more years, $2,000,000. 4 more years. $3,000,000. And so on. By the time you retire you should be a multi millionaire.
Real Estate is okay, but your returns are slow. It will take 20 years to get your rental return high enough to buy another house.
You can buy your own stocks, and try to beat the 10.4% average. But that is highly unlikely and the odds are well against you. Play safe, buy the S&P 500, for a guaranteed safe future. But remember, don’t look at the markets every day, every week or even every year. 5 years is still short term! The real return is made after 10-20 years. Don’t sell because the market goes down, that is the best time to buy. Don’t buy because the market has gone up, that is the worst time to buy. Best way is just buy monthly, otherwise you’ll miss too many opportunities.
Good luck.