I actually just did a "spring fajita dinner" (or "taco" dinner) on Saturday but only for about 6 people. I made enough for about 12 though and here's what I did:
Now, I'm in Minneapolis and have a limited number of Mexican grocers, so I'm sure you can find much more than I...
-Skirt steak - Grabbed this a the Mexican grocery - Arrachera (sp?) - basically skirt steak, sliced very thin. I marinaded in a bit of lime juice. It so thin and grills so fast that I leave it to the last thing on the grill. It basically comes off and is chopped right after so that everything is still warm. I think keeping the meat warm is the hardest part so you may want to think about warming trays or cast iron pans for serving vessels.
-Chicken breast - I pounded these flat - more uniform than anything so they would grill pretty quick. Salt/pepper more lime juice (I'm not a fan of overly seasoned meats since I leverage a few different salsas. These are grilled over charcoal and then "finish" on the cool side of the grill while the steak get's flash grilled.
-Corn tortillas - locally made in Mpls. I just throw these on the grill for a few seconds per side to heat through and then into a tortilla holder (like found in a restaurant). It's the best way I found to keep the tortillas warm and soft for a large party. I do this right before the steak goes on the grill because it goes fast.
-I bought two house-made, pre-made salsas from the grocery store and they were great. One was a salsa verde and the other a nice spicy red sauce - not sure exactly what kind. I also made a pico de gallo for a fresh option as well as a simple guac, which went fast with chips. Also on the table was the (I believe) house-made crema from my Mexican grocer and a really nice fresh cheese I grabbed there as well. Oh yeah, cut up lime, onions and cilantro were in serving bowls as well.
-I did a quick/easy black bean, refried beans with garlic and a little water to thin it out. Basically, two cans of beans, drained; fried with a couple of cloves of garlic. Add water to thin to consistency. Very easy to re-warm right before serving.
-Grilled zucchini and corn. Corn isn't in season but it's available. I thought about the corn/squash, bean thing so I basically grilled the zucchini and corn, sliced off the corn and ate the two with a little refried beans. I like to "dress" my beans when I make my plate so I had the beans, zucchini, and corn with a little salsa, crema, and cheese.
Now, it's all very simple and easy to do - exactly what I like about a party. Is it the most "authentic" - well, no but that wasn't my goal. I like simple tacos with great ingredients and the salsas, guac, and crema were all fresh and I thought it made the biggest difference. Our guests are originally from the bay area and have been craving "good Mexican" for a while. Although, what I made wasn't truly "good Mexican" it did hit the spot and many of the flavors were there. Most importantly, it was pretty easy to pull together.
Have fun!