The trend of money flowing towards robo-advisers and passive asset allocators is one that is likely to continue in the current market environment. Amid the more recent back drop of low volatility capital appreciation in stocks and bonds, it seems that the natural evolution is to put your portfolio on auto-pilot or give it to someone who is just going to rebalance it quarterly.

That’s not necessarily my philosophy on investing, but I understand that some people want to stay fully invested at all times or use continual dollar cost averaging to their advantage. In order to do so, you may find yourself searching for the “perfect investment” or asset allocation to hold for the long-term.

Instead of paying someone to do that for you, I recommend taking a simpler approach that incorporates multiple asset classes through a flexible investment vehicle.

A favorite individual stock such as Apple Inc (AAPL) might be suitable for some investors. However, if you want real diversification, low cost, and a global reach, I would consider using an exchange-traded fund instead.

The following funds represent excellent opportunities for long-term investors.

Vanguard Total Stock Market ETF (VTI)

You can’t go wrong with VTI for complete coverage of the U.S. stock market across multiple styles and market caps. This ETF charges a rock bottom expense ratio of just 0.05% for access to 3,800 U.S.-based publicly traded companies that include large, mid, and small-cap segments.

While that level of diversification may seem overly broad to some investors, it is perfect for those who are looking for a core position in a small account or to simplify many overlapping funds down to a single entity. VTI has over $50 billion in total assets and a current 30-day SEC yield of 1.88%.

The best thing I can say about this ETF is that it is low cost, extremely liquid, and will provide a passive vehicle for tracking the entire U.S. stock market. In addition, it has very little turnover to minimize tax implications for investors that wish to hold this fund in a long-term taxable account.

Vanguard Total International Stock ETF (VXUS)

Often times I see investors trying to get cute with their international exposure to overweighting certain areas of the globe in order to try and capture the best performers. However, if I was to pick just one ETF to bet on the entire market outside of the U.S., it would be VXUS.

This ETF contains over 5,800 securities of both developed and emerging market nations around the world. That includes everything from Canada to China and everywhere in between.   VXUS also follows a passive index approach similar to VTI with a minimal expense ratio of just 0.14%.

Pairing an international fund such as VXUS with VTI can provide you with full coverage of global stocks in just two positions that are easy to track over time. If you wanted to pair that down to just one vehicle you could always select the Vanguard Total World Stock ETF (VT) as well.

PIMCO Total Return Fund (BOND)

Picking a single bond fund to pair with my stock exposure is not an easy task considering the challenges facing fixed-income over the next several years. Ultra low yields around the globe have made the way forward a potentially treacherous one given the expectation of interest rate cycles changing over time.

Despite the turmoil associated with its portfolio manager leaving last year, I would be inclined to incorporate BOND in my portfolio as a solid long-term holding. The reason I selected BOND has to do with a number of complex factors that include the fact it is one of the only truly global broad-market offerings in the ETF space with sufficient size and history.

The newly established Fidelity Total Bond ETF (FBND) may be a suitable alternative. However, I don’t believe that Fidelity has the same level of sophistication when it comes to fixed-income research and security selection as compared to PIMCO.

In addition, I like the active style of the BOND portfolio that gives the manager flexibility to target specific duration, credit quality, country, and sector exposure. I truly believe that these factors are important when competing with passive “total bond market” indexes from the approach of managing risk to enhance returns.

Cambria Global Asset Allocation ETF (GAA)

If you are looking for the structure of a global multi-asset index that takes care of rebalancing your asset allocation, GAA should certainly be on your radar. This “fund of funds” is the first ETF of its kind to offer exposure to stocks, bonds, real estate, and commodities with zero expense ratio.

While there are still minimal “acquired fund expenses” to own the 28 underlying ETFs, GAA is focused on low-cost indexes to achieve its goals. The current asset allocation of GAA is 51% bonds, 43% stocks, and 7% commodities. The majority of the underlying holdings are Vanguard, iShares, State Street, and Market Vectors products.

The benefit to GAA is that their model is created using not just a market cap weighted methodology, but also incorporating value and momentum as well.   The ultra-low fees to own this fund combined with the added bonus of automatic rebalancing make this an attractive position for moderate or conservative investors looking for a long-term core holding.