Investor Bill Ackman runs Pershing Square, a hedge fund with about $11 billion under management. Ackman's general strategy is a combination of value investing and activism -- Pershing Square buys up large stakes in undervalued companies and works with the firm's management to improve the underlying business.

In the second quarter, Pershing Square purchased a large stake in Air Products (APD -2.00%), sold some of its General Growth Properties (GGP), and bought nearly $2 billion worth of calls on Procter & Gamble (PG -0.33%).

Ackman's next war could be with Air Products
In the second quarter, Ackman acquired over 10 million shares of Air Products for his primary fund Pershing Square. He also bought shares of the company in a special purpose vehicle, raising additional funds to push for change.

Air Product's management caught on to Ackman's activities, and adopted a poison pill back in July. If any single shareholder were to acquire more than 10% of the company, existing shareholders would be granted the right to purchase more shares -- thereby diluting Ackman's stake. Consequently, Ackman stopped just short of the 10% threshold.

Pershing Square has yet to make its plan for Air Products known. But if the management disagrees with Ackman, he will likely make his ideas public, as frequently turns to the media.

Pershing Square was forced to fight a proxy contest with Canadian Pacific -- a contest it ultimately won. Pershing Square's new management team has done a phenomenal job of turning around the company, and shares are up more than 44% in the last year.

It's possible that something similar could happen with Air Products. The company hasn't been a particularly good investment in recent years -- since 2010, shares are up 2%, and that includes the recent rally prompted by Ackman's involvement.

If Ackman begins agitating for a change, the company's suffering shareholders could accept his ideas.

Ackman pairs back General Growth
Pershing Square's investment in General Growth has been impressive, to say the least. During the depths of the financial crisis, Pershing Square brought up a large stake in the company at about $0.71 per share.

Those shares are now worth about $20, not to mention the value of the spin-off, Howard Hughes.

But now, after nearly 5 years, Pershing Square may be starting to exit. The firm sold about 9% of its stake in the second quarter.

Last year, Pershing Square and General Growth had a disagreement. Ackman wanted General Growth to sell itself to rival Simon Property Group, while General Growth's management wished to remain independent.

That sale never took place, and Ackman reversed his position earlier this year.

After experiencing a strong rally, General Growth appreciation has stalled. Over the last year, shares are up less than 5%, badly underperforming the S&P 500.

General Growth's spin-off, Howard Hughes, has done far better. Since its creation in Nov. 2010, shares are up nearly 200% -- General Growth is only up 16% over the same period of time.

Notably, Ackman has more control over Howard Hughes, given that he's chairman of the board. Pershing Square's position in Howard Hughes has remained untouched.

Pershing Square rebalances Procter & Gamble trade
Pershing Square rebalanced its ownership of consumer products giant Procter & Gamble in the second quarter, selling off nearly 70% of the shares it owned, but buying $2 billion worth of call options on the company.

Ackman had been urging Procter & Gamble to fire its CEO, Bob McDonald, for months. He finally got his wish when, earlier this year, Procter replaced McDonald with its former CEO, AG Lafley.

Prior to McDonald's retirement, in May, Ackman gave a presentation on Procter & Gamble at the Ira Sohn investment conference.

Ackman argued that the company could be earning $6 per share within two years, giving it an intrinsic value of $125 per share (including dividends). Now trading at $80, that would represent upside of more than 50%.

But not everyone is swayed. Analysts at Standpoint Research downgraded the stock to Sell back in May, arguing that a new CEO would not be able to move the needle.

The biggest factor to watch may be the strength of the dollar. A stronger US dollar weighs on a multinational like Procter & Gamble, as more than 60% of the firm's sales come from abroad.

Nevertheless, Procter & Gamble was able to beat analyst expectations last quarter, posting better than expected earnings.

Investing alongside Pershing Square
Despite recent struggles with Herbalife and J.C. Penney, Pershing Square has one of the best track records in the business.

Investors who bought into Canadian Pacific when Ackman began pushing for a change could've more than doubled their money, while General Growth has offered a nearly 3000% gain since the depths of the financial crisis.

Right now, Ackman appears to betting big on Air Products and Procter & Gamble, while cutting his firm's exposure to General Growth.

While blindly following a fund manager -- however profitable -- into a position might not be the best idea, in light of Pershing's strong, long-term track record, these are definitely investment ideas to consider.