In this hypothetical are we assuming we have control of the house and senate? Or is the house and senate the same as they currently are? Because without total control, nothing will get done.
Assuming total control my to-do list would be:
1) undo everything that newt Gingrich did in the 90's to restore the ability of Congress to pass legislation not written by lobbyists.
2) Enact laws that prevent the president and a complicit controlling party to essentially ignore oversight with no real recourse. Remove all people hired or appointed by trump and anyone his lackeys hired.
3) Implement better oversight of our intelligence agencies so that information like a foreign power trying to influence or elections isn't held by a few people.
4) Campaign finance reform, no private money, no foreign money, completely transparent. Limit the election process to six months, 3 for primaries and 3 for the general election. Enforce the 503c rule that says your primary cause must be for helping the public.
5) Require all military use to get authorized by Congress first with emergency responses to be approved by the speaker and senate majority as well as the minority speakers.
6) Remove sanctions on Iran. Sign the climate accord. End the stupid trade war. Increase sanctions on NK and Russia as well as get the UN and all other countries to agree to such sanctions.
7) Defund the military budget by at least a 1/3.
8) Pass an infrastructure bill as well as a climate change plan that includes better vehicle efficiency standards.
9) Pass some sort of health care reform with price controls and a revamp of IP laws for the pharmaceutical industry.
10) if one doesn't already exist, create a government agency dedicated to science that basically does research for government and for the public good (kind of like the CDC but without the ability to be influenced by politics).
11) Replace "in god we trust" with "in the name of science" or somethin like that.
I'm sure the are more but those are the main ones.