Many people recommend the Vanguard Total Market funds. I have my retirement account, and my son's college account invested in these (although my son's is invested in the 'Fidelity' versions of the same funds):
90%
VTI - Vanguard Total Bond Market ETF (no minimum investment)
VTSMX - Vanguard Total Bond Market Investor Shares ($3K min.)
VTSAX - Vanguard Total Bond Market Admiral Shares (lowest cost, $10K)
10%
BND - Vanguard Total Bond Market ETF (no minimum investment)
VBMFX - Vanguard Total Bond Market Index Fund Investor Shares ($3K min)
VBTLX - Vanguard Total Bond Market Index Admiral Shares (lowest cost, $10K min)