I've always been a fan of the theme party, where the menu, music, cocktails, and decor all play into the same motif. The last big-themed party I had was my "Leon Trotsky in Mexico" party, so called because it fell on a date right between May Day and Cinco de Mayo. While I did play some tango records and mix up a fair amount of Bloody Marys and anything else made with Russian vodka, the party's big attraction was the make-your-own-taco bar. It really is one of the easiest and most crowd-pleasing menus I've ever served. Suggestions for your spread include...

Fillings
Beef. Of course. Just make your standard ground beef recipe with some taco seasoning. I like to add some tomato paste and chili powder to mine.
Pork or Chicken. You can get it ground, but you might want to do these shredded for variety.
Vegetarian. You can go a few ways on this. There's getting your traditional ground meat substitute and spicing the heck out of it. Or you could go into something more bean-based.
Fish or Chorizo. Another option, if you want to go there.

Tortillas
Well, you must at least offer the choice of a hard or soft shell. Beyond that, you can always go with the blue corn hard shells, or perhaps a curry or garlic tortilla. Go to the store, see what they've got.

Cheeses
Naturally, you'll have to have shredded cheddar cheese. Some Monterey Jack or mixed cheeses might be nice also. I recommend getting some cojita or queso fresco for variety. Also because pork tacos with cojita cheese, red onion and pineapple salsa are awesome..

Vegetables
Tomatoes. Diced, of course. Try to get hold of some yellow tomatoes if you can or, failing that, at least cherry tomatoes. some kind of option.
Onions. the standards white or yellow, but also chop up some purple and green.
Jicama. Avocado. Radish. Whatever you like and think might work.
Foliage. Iceberg lettuce will be fine, but if you happen to have some arugula...

Toppings
Salsa. Your standard red, naturally, but do not forget to provide a salsa verde, a tomato-less variety with corn or beans and a fruit with mango and/or pineapple.
Guacamole.
Hot Sauce. You've got lots of 'em in the refrigerator. Also, if you have any taco sauce, mole sauce, or anything else that seems likely, by all means, put it out.