If you don't have an emergency fund, I'd sock that 20k away in Ibonds. You can buy 10k now, and 10k next January. See here for the benefits of iBonds:
http://www.bogleheads.org/wiki/I_Savings_Bonds
If you do have a sufficient emergency fund, put it all in Vanguard Total Stock Market Index Fund Admiral Shares (VTSAX). It's not exactly conservative, but it is tax efficient, and has some of the lowest fees outside of the TSP.
You don't really want to be holding bonds outside of tax-deferred unless you absolutely have to. If you feel like you must hedge, it's better to shift money in your retirement accounts into bonds to offset your riskier taxable investments.