In Highland Park, try Los Mogotes de Michoacan. They meet all your criteria, the setting is festive, and the food is very good, even outstanding for the price and the location. La Casa de Isaac and it's expansion Isaac and Moishe, also in HP, offer somewhat better food, but two or three times the price and not, I think in the event, a "margaritas for Cinco de Mayo" sort of destination, although others might have a different opinion. Los Mogotes also at least for a while was hosting live music, so perhaps they have something special planned for 5 May.
JiLS