I'm not sure why everyone feels compelled to tell you that someday it's conceivable that you might want a traditional instead of Roth 401k/IRA account. It's true, but it's not highly relevant and a Roth is the correct choice for now.
As far as what to put in it: like I said, I didn't think the Asset Allocation of the lifecycle funds was appropriate, so I eschewed them in favor of just getting VTSMX. When your portfolio is that little (and mine is), there's not much to be gained from diversification. Somebody our age would probably be better off with something like a 90/10 or 95/5 stocks/bonds asset allocation, but 100/0 is really pretty close to that.