OK I will start:
Change the President's tenure to one six year term.
Hopefully, this would spare us all the political machinations that go on regarding a reelection campaign. Everyone (including foreign leaders) would know who they are dealing with and would not as tempted to stall things until the next election. Besides, most administrations seem to run out of steam after six years anyway.
Make it a single 8-year term and I'm with you. However, realistically, that only ramps up those who won't accept the results of a constitutional election and they will almost certainly redouble their efforts to un-do it.
For my single constitutional amendment, I am torn between two:
1. Except for general appropriations bills necessary to run day to day operations of each department of government, every bill introduced in either chamber of Congress will be a stand alone bill to be voted up and down on its own merits. Further those in Congress cannot revise and extend their remarks after the debate is closed, and cannot change their vote once the vote is final. No legislation that utilizes the people's money in any regard, requires action by any person or department of government, or that attaches legal requirements to any person will be passed by voice vote alone. Every elected official must be on the record when he/she votes or fails to vote, and all votes will be issued to the media and made public to any private citizen who wishes to see the passed legislation and who voted for it at the end of each week or month.
- or -
2. Congress shall pass no rule, regulation, or law that provides any benefit to any person, group, demographic, or entity that does not provide that benefit to all, and will authorize no grants, benevolence, benefit, or charity to any person, group, demographic, or entity that is not authorized for all. All contracts issued will be put out for bid and issued to the bidder that offers the government and therefore the American people the best deal. Those in Congress will contribute to their private 401ks from their salaries, will buy health insurance on the open market like everybody else does, and will not receive retirement benefits when they leave. In other words, take away the ability of Congress to legislate, buy, bribe, or coerce benefits for itself at the people's expense.