In the past year I have accumulated about $200,000 in "cash" because I am fearful of investing in anything that would have any risk associated with it.
Unfortunately, even "cash" has a substantial risk -- that of losing purchasing power if and when inflation increases. Some of the "early retirees" in this forum seem to be too young to really appreciate the negative impact that inflation can have on the purchasing power of people who are living off of their invested money. Inflation (within reason) is not necessarily bad for the overall economy, but when the inflation rate increases, it shifts purchasing power from creditors (investors) to debtors and people who are working and experiencing the rising wages that accompany general inflation.
Other than a nominal amount that I hold in a checking account for purposes of routine bill payment, I keep "cash" in Vanguard's Short Term Corporate Bond Fund, which historically has provided a return that at least matches inflation, even after taxes on the dividends. (The "dividends" on such funds are actually interest payments from the underlying bonds and therefore are still taxable as ordinary income).
For a greater probable real (inflation-adjusted) return, with minor risk of near-term loss of value, I can't think of a better investment than TIPs. I-bonds pay less interest and are only preferable to TIPs if you are in a high tax bracket, since the "interest" payments on I-Bonds are tax-deferred. I-Bonds are not as liquid as TIPs because there is a mandatory holding period.
You should also consider REITs and high yield bond mutual funds. In terms of the "standard" allocation between stocks and bonds, these may be considered to have risk/return characteristics that are about 2/3 like stocks and 1/3 like bonds, but the price fluctuations will be different than either stocks, bonds, or even a fixed blend of stocks and bonds. Therefore, using these assets as partial substitutes for stocks and bonds can reduce a portfolio's volatility without reducing its expected return.
For example, if my "normal" preferred allocation (outside of my "cash reserve") were 50% stocks and 50% bonds, I might put 10% into REITs and 15% into high yield bond funds, reducing the stock allocation to about 35% and the bond allocation to about 40%. And I would have a substantial portion of the bonds in the form of TIPs.