So, every character starts the first encounter of the day with one action point, X surges (based on class), and one magic item use(MIU). After the encounter they get to pick whether they would like another AP, surge, or MIU? It goes on this way, getting a new choice at teh end of each encounter, until the character takes an extended rest. At which point they are reset back to having one AP, X surges, one MIU...
Be careful, I think that this adds to what Paul Strack was saying about characters hording their resources. If after every fight they get to choose: surge, AP, or MIU, they will just accumulate a surplus of MIUs or surges or AP (depending on the player) and blow them all against the BBEG. You're climatic fight, might not be so...
Also your characters may make easy fights too difficult in an attempt to save all of their resources for later. Rather than rationally thinking: "I'm gonna get another MIU, I may as well use one now..." It may become, "If I don't use my magic item now, I can save it, get another one, and later have two uses against the evil warlord --- 'course, I'm getting my butt handed to me now and could really use that magic item... oh well."