I must admit, I'm not necessarily a cryptocurrency believer, much less a proponent, but I do want to track it and do have it in one of the test portfolios (RPC Growth). This trust, which operates a bit like a closed-end fund but may soon become an ETF, is my preferred vehicle for crypto.
A pillar of Risk Parity portfolios, Long-term Treasury Bonds provide positive expected return, some income, and most importantly, negative correlation with equity funds. To invest in them, I use low-cost, no-frills VGLT, though alternatives such as TLT and EDV are also great choices.
The mainstay of many portfolios, VTI is my choice in the test portfolios when you want just one, all-around equities fund. With an ultra low expense ratio and over 4000 companies inside, this does the job of capturing the equity risk premium in the stock market.
For Commodities, I use PDBC (Invesco Optimized Yield Diversified Commodity Strategy ETF) as my preferred vehicle in the test portfolios, and also hold it in real life. Features: largest "broad basket" commodity fund, decent expense ratio, high dividends. Better for tax-advantaged accounts.
For gold, I use GLDM (SPDR Gold MiniShares) as the representative choice in the test portfolios, and hold it in real life, as well. Low cost (expense ratio: .18%) and simple.