Option 1: High yield savings account such as ING Orange Savings account: http://home.ingdirect.com/orange-savings-account
There are many other options besides ING.
This is the safest and probably best answer. You'll get about the same yield as a 5 year CD.
This is very liquid money with no minimum. Obviously you'll have to check the terms and conditions for the particular option you choose.
Option 2: A bond mutual fund. I use vanguard and have my business starting monies in their Short-Term Investment-Grade bond fund. The symbol is VFSTX and the minimum investment is 3k. The "Admiral shares" VFSUX offer lower fees but you need a 50k minimum investment.
For liquidity, this option also has check-writing privileges but checks must be over $250.