Agree with CD's. Two year CD's look to be yielding about 1.45% right now, which is similar to inflation but with no possibility of loss of principal.
By comparison, Vanguard's short term bond index is yielding 1.35%, with a standard deviation of 1.36%.
Their short term corporate bond fund is yielding about 2.01%, std dev of 1.79%
Their short term treasury fund yields 0.71% with std dev of 0.71%.
Yield is from Yahoo Finance and volatility from Portfolio Visualizer since January 2010. Otherwise high yield savings may be best for greater liquidity. Finally, there is a corporate bond ETF family that holds bonds to maturity in a target year. This is an option I haven't seemed mentioned much, but might be worth a look if you have a known date at least two years in the future.