Retirement is not the time to take on a ton of risk, but younger people with a longer time horizon to invest shouldn't be afraid to dip into at least a few stocks with a spicy risk-to-reward profile. After all, it's often these types of investing vehicles that are most capable of producing market-beating gains. 

Armed with this insight, here is a look at three stocks our Foolish contributors think might turn out to be great buys for investors with a long-term outlook. 

Andres CardenalInvestors in retirement typically look for solid and stable companies, preferably with big dividend yields. TripAdvisor (TRIP 1.19%) does not fit that bill, but the company offers enormous upside potential for younger investors who can withstand high volatility in exchange for big room for gains.

Source: TripAdvisor

TripAdvisor is a leading player in online travel metasearch. The company allows consumers to search for hotel rooms and flights across multiple online travel agencies and get a single list of results. TripAdvisor aggregates reviews and information about all kinds of travel destinations and tourist attractions. The platform features 350 million average monthly users, and it has nearly 290 million reviews as of the third quarter.

TripAdvisor recently made a deal with Priceline to make the latter's platform available in the former's instant-booking platform. The service allows TripAdvisor visitors to instantly make a reservation after comparing prices and available choices, as opposed to sending them to an outside booking service. This deal was a major victory for TripAdvisor, consolidating the company's competitive position in a promising growth business.

Investors are becoming increasingly concerned about the possible impact of terrorism and military conflicts on travel demand. While this could certainly affect TripAdvisor and other industry operators in the short term, the long-term growth story still looks quite exciting. 

Steve SymingtonOn the heels of a solid quarterly report last month from Yelp (YELP -1.43%), I'm optimistic that the local business-review specialist is well-positioned to take advantage of consumers' shift to mobile devices as they shop. Yelp confirmed that shift last week, stating that 71% of all searches on Yelp are being conducted from mobile devices, up from the 70% it reported in Q3. And that makes sense, as Yelp enjoyed an impressive 22% growth in monthly unique visitors on mobile to 89 million last quarter, more than offsetting a 2% decline in monthly unique desktop visitors to 79 million. All told, revenue climbed over 40% to $143.6 million, including a 36% increase in local ad revenue to $115.9 million. Meanwhile, adjusted earnings came in at $2.7 million, or $0.03 per share, well ahead of the $0.09-per-share loss analysts were expecting.

But there are a number of reasons retirees should shy away from the stock. For one, retirees should keep in mind that Yelp still isn't profitable based on generally accepted accounting principles; it technically reported a GAAP net loss of $8.1 million, or $0.11 per share last quarter. That's fair enough, though, as Yelp's efforts to monetize its core U.S. business are still in their infancy and should improve over time.

At the same time, Yelp management has been clear that it will take a number of years for its  international sites to bear similar fruit. As it stands, international sites contribute roughly $3 million in revenue per quarter (or roughly 2% of sales), but expenses related to those international sites are running at roughly $6 million per quarter, creating a significant drag on Yelp's bottom line. Again, however, it's hard to blame Yelp for investing so heavily in fostering its global platform to take as much market share as possible in these early stages. 

Finally, as long as Yelp is unprofitable and growing quickly, its stock tends to suffer significant volatility in the meantime. Despite an 11% pop the day after its most recent report, for example, shares of Yelp still sit around 45% lower than they started this year. In the end -- and keeping in mind that Yelp doesn't offer a dividend to reward investors for their patience -- I think Yelp is best suited to long-term investors with the time to watch its growth story unfold in the coming years. As such, I'm convinced that retirees would be wise to put their money to work elsewhere. 

George Budwell: Highly volatile biotech stocks may not be appropriate investment vehicles for retirees, but younger investors shouldn't entirely shun them. Their market-beating growth prospects make them worth a closer look. The small-cap immunotherapy company Agenus (AGEN 1.05%), for instance, has the potential to double, or even triple, from current levels within the next 12 months, mainly as a result of its emerging immuno-oncology platform. By the same token, this stock could end up muddling along if things don't go according to plan.

Recently, we learned that Agenus now has nine lead checkpoint inhibitors partnered with either Incyte or Merck, along with a host of unpartnered drug candidates. These two partnerships have brought in much-needed non-dilutive funding and have helped the company build a cash runway that easily extends into 2018, according to the company's third-quarter earnings report.

Source: Agenus.

The downside is that the market hasn't exactly figured out how to value Agenus' shares, because the company hasn't been forthcoming with a lead indication for its diverse checkpoint inhibitor platform. As a result, its stock price has been on a roller-coaster ride throughout 2015, and this erratic behavior will probably continue until Agenus names a lead experimental product.

Having said that, Agenus is more than likely going to unveil a cohort of top immuno-oncology product candidates within the next year, given how quickly these types of drugs have progressed in clinical studies in general. And once they do, this stock should react favorably, making it worth a deeper look by risk-tolerant investors.