Like others have said- seriously, if you must buy from Mcdonald’s and you feel self-conscious about it, go through the drive through, and ask them to put it in your own hotcup. Some places do that, and you actually save money that way too!
Here’s another good question- if on your way to shul, there is a very good coffee shop you enjoy, or say a duncin donuts which is kosher, and you arrange before shabbos the payment and all- are you allowed to enter the store on shabbos and come out with a hot cup of coffee? Provided there is a Eruv in your town.