"Tell us about yourself" opening question:
I never "sold myself" too much in this response. I didn't really even tie my interests steadfastly to the school this early. This was a question where I made an honest effort to show that I am a "person" too. I told them where I was from, what school I attended, what I majored in, sometimes mentioned my family, told them about a few cool extracurriculars I did (and showed that I really enjoyed them), I sometimes quickly tied relevant activities back to the school I was interviewing at (A VERY small nod), told them about my interests outside of school, sometimes mentioned a few interests (like research) that were somewhat academic. Load them up with things to talk about and be enthusiastic. Imagine if your new girlfriend's mom asked you this question... answer it like you were talking to her. This is not the time to be competitive.
"Tell us something interesting about yourself" mid-interview question:
Really, I just told them something quirky I was/am passionate about. Luckily I have some unique things I really enjoy that tend to get a chuckle, but my passion for them makes it a legitimate answer. For questions like this I was never too serious.
Piece of advice: I never tried to "sell myself" in the way I feel most people view it on SDN. They already think you are qualified, so selling yourself relative to others (unless there is a blemish they ask about) is probably futile. I just stayed committed to showing them who I was, highlighted characteristics/experiences I thought were important to have entering medical school, and expressed why I thought I would fit there (both directly at times and passively at other times). So I guess my moral is don't try to "sell" yourself as much as you are just "being" yourself and showing them why you are a good fit for them. AKA Relax.
I was accepted to every school I interviewed at, so this must not be totally off base...

Good luck and feel free to PM me if you need any help.
P.S. In my opinion, one should never even get close to narcissism. Honestly, I rarely even get close to "selling myself" with too direct of wording. The characteristics they are looking for should be seen in the actions and experiences you talk about. Sometimes you can note that you think "this or that" will help you moving forward. I would never say "this shows that I am hard working". Rather I would talk about my engineering background and show that I developed a work ethic. I may, MAY, say that I feel this will help me throughout my career if I felt it flowed well and was appropriate.