It doesn't get more classical than Mozart. I picked a short youtube thing because it was quick. I have it in my collection from The Academy of St Martin in the Field and probably a couple more that I'm not thinking of. Many other versions of this and other Mozart pieces are available on the Internet, I would urge you to listen to many of them and add appropriately to your own collection.
"Eine Kleine Nachtmusic" just makes so much sense to me. I guess uplifting is what comes to mind more than anything else. It really improved my mood to listen to it this morning.