Quitting smoking does not cause weight gain. Only overeating, not eating healthily and lack of exercise cause weight gain. Do things that keep your mind off of smoking. Go to the movies. Take a walk, suck on a lollipop, go to museums or other places that don't allow smoking.
Swimming is a good idea.
Quit smoking one day at a time and think only about the part of the day that you are in. I'm not going to smoke before noon. Sometimes just do it one hour at a time. This is a lot easier than trying to stop forever. You can also stop by the Wellspring Resource Center in Chapin Hall - Room 407 to see more information, literature, and Nicotine Anonymous Meeting schedules.