Lots. The POTUS (President of The United States) can't do much without the very elite membership of the Senate. He or she needs two-thirds of 100 votes to do anything important. Right off the bat, the president has to name and appoint hundreds of the highest ranking federal employees to lead all the committees that do the heavy lifting of law making. Not only will you be helping that college student back home get her passport through the necessary red tape in time for her Peace Corps job in Ecuador, you will be hiring the people responsible for the national budget, foreign affairs, the environment, health, education, labor, veteran's affairs, banking, agriculture, energy, and Indian affairs.
In other words, you have the power to change the lives of 311,591,917 people even if some days you're so stressed out you can't decide between paper or plastic.