Many investors don't think they can start investing with just $100. After all, a single share of Apple (NASDAQ: AAPL) costs about $130, while one share of Amazon (NASDAQ: AMZN) costs nearly $3,500. That was certainly the case when I started investing many years ago, when brokers scoffed at the notion of selling fractional shares and charged commissions of about $10 per trade. But today, most brokerages offer fractional shares and commission-free trades -- which makes it easy to get started with $100. It might be tempting to take that cash and chase some of Reddit's hottest meme stocks, but here are three better ideas -- and some of these investments could generate much bigger gains than you expect. Image source: Getty Images. 1. An S&P 500 Index Fund or ETF An index fund, which passively tracks an index like the S&P 500, is a reliable way for conservative investors to build their wealth. Most index funds require minimum investments of a few thousand dollars, but the Schwab S&P 500 Index Fund (NASDAQMUTFUND: SWPPX) lets investors get started with just $1 and charges a low expense ratio of 0.02%. Investors might also consider buying an S&P 500 ETF (exchange-traded fund) that tracks the market. Unlike index funds, which can only be bought or sold once a day, ETFs actively trade like stocks throughout the day. The Vanguard S&P 500 ETF (NYSEMKT: VOO) is a popular choice, since it charges a lower expense ratio (0.03%) than most other S&P 500 ETFs. Investors might think this approach generates low returns, but here's how these funds actually fared over the past three years -- assuming you reinvested their dividends to boost their total returns. Source: YCharts Past performance never guarantees future gains, but the S&P 500 has generated an average annual return of about 10% since 1926. That easily beats savings accounts, which have an average interest rate of 0.06%, and 12-month CDs, which offer an average interest rate of 0.14%. 2. Blue-chip dividend stocks Investors who are willing to take a little more risk should buy shares (or fractional shares) of reliable blue-chip dividend stocks. One solid example is Procter & Gamble (NYSE: PG), the consumer staples giant that sells Tide, Charmin, Pantene, Pampers, Tampax, and other well-known brands. P&G is so good at generating cash that it's raised its dividend annually for 65 straight years. That streak makes P&G a Dividend King, or a member of the S&P 500 that has raised its dividend annually for more than half a century. It currently pays a forward dividend yield of 2.6%, which is much higher than the 10-Year Treasury's current yield of about 1.5%. P&G experienced unusually high growth throughout the pandemic as shoppers stocked up on essential products, but analysts still expect its revenue and earnings to rise 6% and 10%, respectively, this year. P&G still trades at a reasonable 22 times forward earnings -- even after generating a better total return than the S&P 500 over the past three years -- so it's still a great stock to buy and forget. Source: YCharts 3. Take a chance on growth stocks Last but not least, investors who aren't afraid to lose most of their investment should consider putting $100 into some promising growth stocks. I'm not telling you to chase Reddit's latest obsession or speculate about upcoming short squeezes -- I'm asking you to research some disruptive companies. For example, the cybersecurity company CrowdStrike (NASDAQ: CRWD) dazzled the market for three simple reasons: It provided a cloud-native platform in a market that still mainly used on-site appliances, it was growing at a faster rate than its peers, and its net retention rates -- which measure its year-over-year revenue growth per customer -- consistently remained above 120%. CrowdStrike's revenue rose 93% in fiscal 2020, grew 82% in fiscal 2021, and is expected to climb another 56% this year. The company also turned profitable (on an adjusted basis) last year, and analysts expect its earnings to grow 44% this year. Here's how the market rewarded CrowdStrike over the past year. Source: YCharts But there's the catch: CrowdStrike's stock has been expensive ever since its IPO, and it remains pricey at 345 times forward earnings and 40 times this year's sales. Investors who chase CrowdStrike -- and other similar growth stocks -- should expect a lot of volatility. But over the long term, these growth stocks could generate much bigger gains than the S&P 500 or Dividend Kings like P&G. The key takeaways Investors shouldn't underestimate the power of a $100 bill. It's enough to get your feet wet in the stock market, and there are still plenty of choices for investors -- regardless of their appetite for risk. Find out why CrowdStrike Holdings, Inc. is one of the 10 best stocks to buy now Our award-winning analyst team has spent more than a decade beating the market. After all, the newsletter they have run for over a decade, Motley Fool Stock Advisor, has tripled the market.* They just revealed their ten top stock picks for investors to buy right now. CrowdStrike Holdings, Inc. is on the list -- but there are nine others you may be overlooking. Click here to get access to the full list! *Stock Advisor returns as of June 7, 2021 John Mackey, CEO of Whole Foods Market, an Amazon subsidiary, is a member of The Motley Fool's board of directors. Leo Sun owns shares of Amazon and Apple. The Motley Fool owns shares of and recommends Amazon, Apple, CrowdStrike Holdings, Inc., and Vanguard S&P 500 ETF. The Motley Fool recommends the following options: long January 2022 $1,920 calls on Amazon, long March 2023 $120 calls on Apple, short January 2022 $1,940 calls on Amazon, and short March 2023 $130 calls on Apple. The Motley Fool has a disclosure policy.Source