In the US, executive orders cannot change tax law, since the "power of the purse" is constitutionally reserved for congress.
If you're thinking of the Iran nuclear deal, that's head-of-state stuff where the president is considered to have more powers (though of course it still was never a treaty, so could be/was scrapped easily by the next administration).
They can make all the agreements they want, but it's not a legal treaty until 2/3 of the Senate agrees, and even then, this stuff requires that laws be passed -- many laws affecting jurisdiction, accounting standards, and the tax laws themselves. None of this can be done with an executive agreement.
> They can make all the agreements they want, but it's not a legal treaty until 2/3 of the Senate agrees
That's not true under international law. The confusing thing here is that "treaty" means different things under international law and US law.
Under international law, any legally binding agreement between two countries is a treaty.
Under US law, there are three types of agreements between the US and foreign states (or international organizations): treaties, congressional-executive agreements, and sole executive agreements. The first are approved by two-thirds vote in the Senate, the second by an ordinary Act of Congress, the third by the President acting alone (without Congressional involvement.) But all these three are considered equally to be treaties under international law. The distinction between the three is purely a US domestic law distinction. Article 46 of the Vienna Convention on the Law of Treaties says that domestic law does not determine the validity of treaties under international law unless the violation is manifest, which means that for most purposes the rest of the world can just ignore this US-internal distinction.
The authority to ratify treaties, in the international law sense of "ratify" and "treaties", solely belongs to the President (and the Secretary of State, and ambassadors, acting on the President's behalf). When the US constitution speaks of "ratifying" a treaty by the Senate, that is not ratification under international law. That's actually a domestic US legislative procedure which confusingly happens to have the same name.
> many laws affecting jurisdiction, accounting standards, and the tax laws themselves. None of this can be done with an executive agreement.
In practice this will likely be done by an ordinary Act of Congress (a "congressional-executive agreement") which only requires an ordinary (not two-thirds) vote in the Senate.
However, one needs to understand that ratifying a treaty under international law, and passing domestic legislation to implement it, are independent things. Under international law, the President or Secretary of State can legally submit the instrument of ratification for the treaty even if Congress hasn't passed any implementing legislation. International law doesn't care about implementation legislation, that's a domestic law concern. Now in practice the President or Secretary of State wouldn't do that, because that is not the traditional practice of the US. But other countries in the world do sometimes ratify treaties before the implementing legislation is passed. That generally happens in systems – whether Westminster democracies or non-democracies – in which the executive can be confident they'll get the implementing legislation passed.
And those executive agreements have no binding legal force, and can be broken by the next executive (or even the same executive who made them) on a whim. See, for example, the Iran deal and the Paris climate deal.
I agree with your main point but I wouldn't say executive orders lack binding legal force. They derive binding legal force from congress or the constitution first telling the executive branch "you go figure out the details here."
Foreign policy.
The SEC.
Heck, the emancipation proclamation was an executive order. Everyone knew Lincoln and his contemporaries wanted to abolish slavery, but Lincoln was absurdly careful at the time to frame the proclamation as a wartime measure aimed at crippling the south's economy. He went out of his way to appeal to existing commander-in-chief powers in order to make it lawful.
