Good farewell/thank you song?

Hi i was wondering if anybody could think of a good farewell/thank you song? See two particular community members have been supporting our high school band for a long time but for personal reasons can't continue to do so. Us senior members of the band would like to perform a song for them at out end of year gala concert especially for them to say thank you for all the work they have done for us. Any ideas?

how about "time of your life" by greenday?? some of it sounds pretty orchestral and the chorus goes "It's something unpredictable, but in the end it's right.
I hope you had the time of your life" so it seems pretty appropriate...they actually play it at a lot of graduations