Book two roundrips:

SFO-JFK on PS. Sunday through Tuesday night or Wednesday morning. Early east coast flights to the west coast get you back early enough in the morning.

SFO-DEN. Fly either Wed night or Thur morning depending on what time you need to be in DEN.

Just tell United.com you don't want to connect. Or perhaps direct SFO-JFK is sold out in which case they are routing you through LAX. LAX-JFK is PS also.