When applying to a college that majors in music what would be some good things involving music I should include in my application?

Any original compositions, or audio clips of your performances, would be good.

Note that the music departments at many colleges that you apply to may give a recommended list of what to submit, and you can contact that department directly and ask them as well...

If you are a trained musician with exceptional talent, submitting compositions or recordings with your application would certainly benefit your candidacy. If you are passionate about music but don't believe that your current level of musical talent would improve your candidacy, then it may be advisable to avoid sharing such recordings. You should certainly seek second and third opinions on this - it really depends on how strong you think your musical talent is.

In the absence of recordings or compositions, you may still convey the strong sense of passion you have for music by showing evidence of internships you have completed in the industry, any awards or articles which relate to your activities with music, or any after school or weekend / summer programs you have been involved with related to music. For example, if you created a program to collect unused musical instruments to give to young children in need, that would be a noble purpose and a great example of a dedicated effort which could convey your drive, ethics, and ability to achieve significant accomplishments. That it pertains to the field of music would convey those your passion for music along with those other fine qualities.

You can read Cornell's undergraduate guidelines for applying to join their music program here: http://music.cornell.edu/undergraduate/admissions/

List any kinds of music lessons you've taken over the years; any music activities you participated in or out of school; how many instruments you play. Of course, you didn't mention what type of music major you want to be - instrumental, vocal, or more music business oriented. So it depends on what you want to pursue.