Spread it out among a whole bunch of different types of investments -- including stocks, bonds, perhaps a precious-metals fund for a small portion of your funds, if you have the ability to do so.
Your statement that you are "not allowed to directly invest in stocks, bonds or commodities" tells me that your 401(k) plan is managed by someone who is responsible enough to know what your retirement plan is for -- and what it's not supposed to be for.
The fact remains ... right now, all kinds of investments suck. So I dont consider being in all bonds to be that bad of an investment at the moment. Esp since it is inflation protected.