Well the OP posed a good question that many of us are seeking the answer to.
However, the real concern here is: What does "reasonably safe principal" actually mean to each of us, and how do we apply it to our individual situations.
As most of us know, anything other than CD's and savings bonds, such as stocks, ETF's, and bonds, carry a certain degree of risk, and the greater the risk, the greater the reward.
I happen to like stocks like Chevron (CVX), ExxonMobil (XOM), McDonald's (MCD), and agree that there are some nice mutual funds and ETF's that also offer decent dividends; however, they do carry risk of principal loss in a down market, so time horizon is important.