There is no need to "dress up" for this day, but if you do, do not wear a sombrero, mariachi suit, serape, fake mustache or anything of the sort if you are not a member of that culture. Those things have historical and cultural significance, and donning them just for a day caricatures and stereotypes people. That's not fun.
Go out and get drunk
There is nothing wrong with drinking in moderation and doing it socially, but responsibility is key. What is the use in celebrating a day if you get sick or can't remember it?
Make English words Spanish by adding an "o" on the end
Not only does it not make any sense, but by doing this, it makes fun of another language and turns it into a joke. The same goes for plays on the holiday name, so no parties or themes like "Cinco de Drinko."
You can make a margarita cupcake or a fun cocktail, or have dinner at a family-owned Mexican restaurant. There are plenty of ways to celebrate Cinco de Mayo without doing any of the three above.