Generally speaking, retirees should err on the side of caution when managing their money. That means a good portion of their net worth should be allocated to low-risk assets like bonds and cash, while a smaller portion should be invested in stocks. That said, buying individual stocks may be too risky or require too much research for some retirees.

Fortunately, there is another option. Index funds are a great way to gain exposure to the stock market while minimizing the risk and work involved. In fact, Warren Buffett once told Vanguard founder Jack Bogle that index funds are "the most sensible equity investment for the great majority of investors."

With that in mind, these index funds could make retirees richer over the next decade.

1. Vanguard S&P 500 ETF

The Vanguard S&P 500 ETF (VOO 1.08%) tracks the S&P 500, an index containing 500 of the largest U.S. companies that covers approximately 80% of the market capitalization of all publicly traded companies in the U.S. To that end, the S&P 500 is often viewed as a benchmark for the entire U.S. stock market.

Sector breakdown: The S&P 500 includes companies from all 11 market sectors, though five sectors account for 72% of its total weight: Information technology (27.3%), healthcare (14.1%), consumer discretionary (11.4%), financials (10.9%), and communications services (8.4%). Its three largest holdings are Apple, Microsoft, and Amazon.

Past performance: The Vanguard S&P 500 ETF has generated a total return of nearly 220% over the last decade, which is equivalent to an annualized return of 12.3%. At that pace, an initial investment of $10,000 would grow into $31,900 over the next decade.

Beyond its broad scope, the Vanguard S&P 500 ETF is a particularly compelling investment for two other reasons. First, the S&P 500 has recovered from every past downturn, and the index generated a positive return 94.1% of the time over all 10-year periods between 1926 and 2017. Second, it bears an expense ratio of just 0.03%, meaning investors would pay $3 per year on a $10,000 portfolio.

2. Vanguard Dividend Appreciation ETF

The Vanguard Dividend Appreciation ETF (VIG 0.29%) is designed to track the S&P U.S. Dividend Growers Index, which includes 289 U.S. companies that have increased their dividend payments each year for at least 10 consecutive years.

Sector breakdown: The S&P U.S. Dividend Growers Index includes companies from 10 of the 11 market sectors (real estate is the one exclusion), and the top five sectors account for 80% of its total weight: Information technology (23.4%), healthcare (15.6%), financials (14.7%), consumer staples (13.6%), and industrials (13.3%). Its three largest holdings are UnitedHealth Group, Microsoft, and Johnson & Johnson.

Past performance: The Vanguard Dividend Appreciation ETF has generated a total return of nearly 193% over the last decade, which is equivalent to an annualized return of 11.3%. At that pace, an initial investment of $10,000 would grow into $29,100 over the next decade.

Beyond its broad scope, the Vanguard Dividend Appreciation ETF is a compelling investment for two other reasons. First, companies that consistently generate enough cash to raise their dividend tend to have strong fundamentals, and that often coincides with share price stability during periods of market volatility. In fact, the Vanguard Dividend Appreciation ETF is down only 5.6% over the past year, while the broader S&P 500 has fallen 10.1%. Second, the ETF bears an expense ratio of 0.06%, meaning investors would pay just $6 per year on a $10,000 portfolio.

As a final thought, retirees should keep at least two years' worth of cash on hand to cover living expenses, though some experts recommend a five-year cash cushion. Additionally, any money retirees will need in the next decade should not be invested in the stock market.