Mark Matson encourages investors to look beyond the S&P 500 and tech giants like Apple (AAPL) and Microsoft (MSFT) to achieve true diversification. Instead, he suggests considering microcap stocks, small-value stocks in the U.S., and large international value stocks. He warns against, what he considers "toxic investments" like cryptocurrency, including Bitcoin (/BTC), and hedge funds, citing high fees and low returns.