Though the above combination of stocks and bonds is perfectly reasonable, I like to diversify further into a few other categories. Mainly I add the following, either in ETFs, options, futures, or physical assets where applicable:
These in sum do not exceed about 8% of my portfolio. I reduce my bonds allocation by 8% to make room for these. The reasoning is to keep some investments non-correlated with stocks/bonds. SGOL and SIVR have lower expense ratios for buy and hold, but also lower options liquidity.
Schwab ETFs do not have much option liquidity. For such cases, I might use different ETFs or futures to represent the same category. Sometimes it is also cheaper to purchase ETFs with a higher price due to trading fees being based on number of shares. I also may use mutual funds if I don't plan to do much active trading. Some equivalents: