Feel good about yourself and convey that feeling to the interviewer. You can be positive about your accomplishments without sounding conceited. Interviewers expect you to say good things about yourself. Be sure to follow-up after the interview. Interviewers, like everyone else, appreciate being thanked for their time.
Colleges notice the gesture and it helps to reinforce a positive impression.
If you enjoyed your interview and visit, find out about opportunities for you to return and to stay overnight on campus, sit in on classes, or attend upcoming special programs. Remember that every college or university wants you to leave your visit feeling good about the experience. If you take the time to research colleges, and then do a little thinking about how you've spent your time and what is important to you, you'll find your visits both informative and enjoyable.