Leaving Song for My Friends?

I've been studying at a small liberal arts and science college for a year now, and I made such good friends. However, I decided to quit next year and study medicine, cause in the end that's what I want to do. So, as some kind of thank you/farewell (although I'll visit them often) I want to sing a song for them during an open mic night where I always perform. So can you please help me with any good and nice songs? (I'm a mezzo btw) Thank you so much :)