I'd recommend "VTI" (Vanguard Total Stock Market ETF). If you can't decide what to buy - buy it all. The "total stock market" fund holds over ~3600 stocks at their market weights, and it's expense ratio is just 0.04%.
But you could even pick a Vanguard ETF at random, and likely do better than cash. Investing 100% in cash will almost certainly delay your retirement. Cash at best roughly keeps up with inflation, but your money doesn't grow in real terms. If you have 1000 hamburgers worth of cash today, you likely will have 1000 hamburgers worth of cash years from now.
Besides your 403(b), if your salary is below the IRS limits, you can also open an IRA and contribute to that.