He'd have been President of Israel.
Does the President of Israel actually do anything?
Not really. The President of Israel is an exceptionally ceremonial role, and they're basically just a symbol of the State. Think the British monarch in terms of actual power.
Einstein as President of Israel won't change
that much, he was already in poor health by 1952 and he died in '55. The most that changes is a certain bit of prestige of having Einstein as president and a bit more international attention paid to the Hebrew University of Jerusalem (he was one of the main contributors in the 1920s to it, and even was on the HUJ's first Board of Governors alongside Chaim Weizmann, Sigmund Freud, and Martin Buber).