Uhm.. not much,
a bottle of water, cheap water!
3 to 4 (if cheap) Brötchen, usually 25c per otherwise you take old ones
Not a beer (at least 1,20€ if you're really lucky, otherwise around 1,60€), not a bread (3-4€), not a Cheese Burger (usually 1,35€), not a chocolate bar (should be over 1€)
Oh, you could beg! Or if you live near a McDonalds you could get a Hamburger.
ya, no coffee. At least not proper coffee
1 Euro? Nothing. You literally can't buy anything with 1 Euro.
Oh wait, you can buy a poststamp, only 62 to 85 Cent.
So, what can you do? Send a letter to someone. Thats pretty much it.
Will be difficult if you're not prepared. You need paper and a pen as well to write a letter. Or a postcard and a pen, but postcards are above 1€