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. The options columns in the tables below have higher option liquidity, however their expense ratios might be higher for buy and hold.
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.