When you're on a budget, @100montaditos is the way to go. They apparently have 100 flavors of their mini toasted buns which come sweet or savory. Most are 1€ each with more premium flavors costing slightly more.
I had squid ink calamari and a pesto goat cheese which was frankly damn good. The buns are toasted to crispy, messy perfection so you kinda need some strong teeth. Bro told me they usually eat about 5 for a proper meal. Compare the size of one bun to the size of the regular-sized chips and you get how mini these are.