I recently sent a thank-you note to someone, and they told me they were very pleased that I did not phone, voicemail, instant-message, or e-mail instead.
A handwritten thank-you note is a mark of appreciation and acknowledgement of the other's effort. During the few minutes that it takes to write and mail it, I get to think about how one more person has helped me on my way to med school. Sending out that good positive energy is always a good thing.
Get some good notepaper and matching envelopes, and a big pile of stamps. A smaller size paper means you don't have to write much to fill the page. If you have time on interview day, you can always write notes after your interviews and then just drop them in campus mail. Otherwise, write and mail at the airport on your way out.