The line item veto is probably the quickest to implement and most useful, as I believe it can be implemented by statute. Term limits would probably take a Constitutional Amendment.
And I would qualify the ending of federal government pensions to elective (and possibly appointive) positions. Leave them in place for the civil service positions.
your wrong already been to scotus and shot down. however there is a way to recreate a virtual line item veto.
there was a virtual line item veto up until Nixon
https://en.wikipedia.org/wiki/Impoundment_of_appropriated_funds
impoundment is an act by a President of the United States of not spending money that has been appropriated by the U.S. Congress. Thomas Jefferson was the first president to exercise the power of impoundment in 1801. The power was available to all presidents up to and including Richard Nixon, and was regarded as a power inherent to the office. The Congressional Budget and Impoundment Control Act of 1974 was passed in response to perceived abuse of the power under President Nixon. Title X of the Act removed that power, and Train v. City of New York (whose facts predate the 1974 Act, but which was argued before the U.S. Supreme Court after its passage), closed potential loopholes in the 1974 Act. The president’s ability to indefinitely reject congressionally approved spending was thus removed.[1]
since its a law, it can be repealed unlike the line item veto which, as with term limits requires a constitutional amendment.