firstly...PLEASE dont have taco shells! go to a local tienda and get a rack of fresh corn tortillas and keep them fresh in a little steamer.
have some good refried beans on hand
try spiced, pureed calabaza (pumpkin) as well...vvvveeerrry tasty!
have real crema instead of sour cream
have queso fresco
have fresh leafy cilantro or culantro, and epazote.
spicy grilled shrip
grilled fish in chunks
pickled red onions
grilled pineapple sticks
There's a chow entertainment item about a burrito bar that might give you ideas.
The tacos that you get from taco trucks and similar establishments consist of a couple of overlapping small corn tortillas (soft), topped with seasoned meat of your choice, and a 'relish' of diced onion, and cilantro. The meat usually doesn't have a lot of sauce. Typical choices are pork, thin steak strips, diced tongue, fatty beef (cheeks), tripe, intestines, chicken, shredded beef.
Avocado is a good side. Chopped cabbage is an alternative to iceberg. A dry, salty, crumbly cheese is an alternative to cheddar (Mexican or feta).
Fish tacos often use cabbage, and a spicy mayo based sauce.
It might be a longshot but it would be really special to have someone there making and grilling fresh tortillas for the tacos. Or alternatively, to have one of those simple tortilla presses, masa dough and an electric griddle so people could make their own.
As for fillings, marinated seasoned shrimp, scallops, or firm fish bring up the level a bit. You also want a variety of salsas -- a hot salsa roja, a mild, and a green salsa verde. A pineapple or mango salsa can also add choices. If there is a good Latin market nearby, you may get some good ideas, such as cactus. Ground beef usually goes in crispy, fried taco shells -- carne asada, al pastor, carnitas, chicken, and even more exotic parts go in soft tacos. Chorizo and cheese can be good, so can a chile rellano or even potato. Besides chopped raw onion, peppers and scallion, grilled onions, peppers, and green onions are tasty. Good luck.