If you are looking for a set-it-and-forget-it way to invest for retirement, here are a few possibilities to consider.

Target Date Funds. So-called target date funds, which match your projected retirement date with a gradually shifting allocation of assets, are a great one-stop option for a lot of people. You invest in the fund, and the fund company changes the allocation as you get closer to retirement.

One example is Fidelity's Freedom Fund series. Fidelity has funds set up in 5-year increments--Fidelity Freedom 2040 (FFFFX), for example. For investors who don't plan to retire until around 2040, the fund starts out with about 63% in U.S. equities, 27% in international equities, and 10% in bonds. As the year 2040 approaches, Fidelity gradually modifies those percentages, reducing the equities and increasing the bonds. All you need to do is keep contributing, as Fidelity handles the rebalancing and reallocation.

Large fund companies like Fidelity, Vanguard, and T. Rowe Price offer such funds, and they are often available through 401(k) plans.

This is a mostly set-it-and-forget-it way to go. But you'll still want to check in on your plan occasionally to make sure you're on track to achieve your goals. There are a number of tools available to help you with this. One way to answer that all-important question, "What is the probability that I will have enough money in retirement," is to run a Monte Carlo simulation on your entire investment plan--not just the investment assets like the Freedom fund mentioned above, but also on spending, income sources, tax rates, and more.

Indexing. Investing in as few as three or four indexed mutual funds or exchange-traded funds (ETFs) would work for a lot of people, especially those still far away from their eventual retirement date. Index funds famously have very low fees and, while by definition you won't beat the markets with a market index fund, you will probably only be slightly behind them due to those fees.

Here are some mutual funds and ETFs to consider, starting with higher-return, higher-risk options, and gradually getting more conservative:

For most people, just as with target date funds or any other sort of investment, the earlier you are in your working and saving years, the higher the percentage of your investments that should be in invested in equities (the first two options above). The closer you get to retirement, the higher the percentage that should be invested in fixed income (the latter two options above). There are a wide range of opinions on what percentages are appropriate. It's partially going to depend on your stomach for market volatility.

Vanguard is the asset management company best known for indexing, but there are plenty of other options. Your 401(k) should have some indexing options available. (And if it doesn't, ask your HR person about getting some added!)

Dividend Payers. At some point, most investors will depend on at least a portion of their retirement savings to generate income in some way. Individual stock investing isn't for everyone, and can't exactly be called set-it-and-forget-it. But if you have the aforementioned stomach for the aforementioned volatility, it's not difficult to assemble a diversified portfolio of stocks that have consistently made dividend payments (and limited partnerships that consistently make distributions) yielding 4% or thereabouts. There are a number of companies that historically have paid consistent or growing dividends through all kinds of markets. Here are a few that we recommend:

As you get closer to retirement, it may make sense to have a mix of these options, depending (once again) on your tolerance for volatility and your need for income. Regardless of what combination you choose, though, you'll want to make sure you've got a solid handle on what your retirement income needs will be. Using an accurate retirement withdrawal calculator is critical for getting a good estimate of those needs.

The investment portion of a retirement plan need not be complicated. In fact, it should not be complicated. But it does need to be monitored, and using the proper tools for doing so is an essential part of a simple plan.