Ain't no party like a taco party

Happy Birthday, Imogen! Let us know how your party turned out.
BB

You should make extra corn tortillas in case there are any guests with gluten or other wheat issues . . . . ;o)

for dessert, try a coconut tres leche cake with rum

I recommend potato tacos like this one from Saveur: http://www.saveur.com/article/Recipes/Tacos-de-Papa-Potato-Tacos

In terms of preparation plans... Here's a breakdown:
My party is on a Saturday night. I'll make all the salsas and the choco tacos and paletas on Friday.
Saturday daytime I'll do a big tortilla production in the morning. I will also marinate the fish and cauliflower in achiote and prepare the tres leches cake.
My partner is in charge of the grill, when guests start to arrive. She will be grilling hangar steak and green onions.
Kiddo will be put to work making the feta and watermelon and mint on toothpicks. In the afternoon I will prep the salads and calabacita and ceviche, and do mise en place for the street corn.
Best friend is in charge of bartending and music.
Expecting about 20-25 guests, including 5 kids under 6.

Imogen, that menu sounds great. I would like to copy portions of it...I wondered if you would share some of your preparation plans and serving plans?

Here's my updated menu:

assorted Mexican beers
margaritas
Jarritos

Guacamole and chips
Feta, mint, and watermelon on toothpicks
Ceviche in shot glasses
Homemade salsas (pineapple; watermelon and jicama, standard red, standard green)
Fish tacos al pastor style with homemade corn tortillas
Tacos carne asada with homemade flour tortillas
Calabacita style tacos with optional black beans
Roasted cauliflower tacos with optional black beans
Street corn
Carpaccio style heirloom tomatoes with avocado and tomatillo salad and fresh herbs

Homemade choco tacos
Peach and blackberry tres leches cake
horchata paletas (for vegans)

Frozen dessert, such as paletas, shave ice or ice cream with Mexican flavors?
https://www.barnesandnoble.com/w/paletas-fany-gerson/1101002592?ean=9781607740353
https://www.barnesandnoble.com/w/my-sweet-mexico-fany-gerson/1102115828?ean=9781580089944

I usually grill marinaded portobello mushrooms as a vegetarian/vegan taco filling option. Pre-steam the caps, use whatever marinade you prefer and do the final grill to heat up, get some color on them, very low maintenance.

Consider other grilled veggies as well: eggplant, romano beans, and zucchini/squashes are three off the top of my head. Slightly more prep work, but still pretty easy.

You could also grill scallions or maybe use caramelized onions, but those take more work.

Tres leches cake is a good canvas for seasonal fruit, especially berries. It's easy to make in large portions and it seems like a relaxed setting, so I think you're pretty set there. If you have the time and inclination, you could look into making some cajeta or dulce de leche ahead of time. It goes well with tres leches and berries.

A chile spiced flourless cake with berries and cream would fit (loosely) the theme.

Pulling on corn as an ingredient, a sweet corn cake with blueberries and honey would (very loosely) fit the theme.

Incorporating ingredients like coconut or lime into something more European like a pound cake would bring in the Mexican theme. If you want to be ala minute and grill the pound cake and serve it with whipped sour cream and mango or something....that would be good. More work, but impressive.

One thing I've learned over the years is, if you are trying to stick to a theme and nothing seems to resonate or fit, just forgo the theme. Its much more important to have a killer dessert than to rigidly stick to a theme. Even a small detail (like marigolds for decor, or some lime zest) can tie whatever you make to a theme. You can stretch the theme as well and make something that's Southwest American or South/Central American. In the end, nobody is going to judge you if you have incredible Mexican food with an incredible Italian dessert, for example.

Roasted or grilled cauliflower makes a good taco filling, especially doubled up with some beans (black, pinto, whole or refried is up to you). I usually like to keep the seasoning on taco fillings simple, since it will be sauced with salsa (and if you're me, hot sauce also). Probably just salt and oil for the cauliflower, or if you want to make it a bit more flavored toss the cauliflower with some of the achiote paste you're using for the al pastor tacos before roasting it. Achiote-rubbed grilled tofu is also pretty good -- just make sure the pieces of tofu aren't too thick, since the flavor won't sink in very far, and unseasoned tofu can be aggressively bland.
For the beans, I strongly recommend cooking your own; it's easy and the flavor and texture really is better. Rancho Gordo beans are awesome if you can get your hands on them, but otherwise just make sure you buy ones that haven't been sitting around for years. I generally cook them with a smashed clove of garlic, a few branches of parsley or cilantro, a glug of olive oil, and salt (about 3/4 tsp per quart of cooking liquid works for me). Left whole, or mashed and refried, they would be a great foil to your salsas.
Other veg. taco filling ideas: roasted/grilled/sauteed mushrooms, grilled zucchini
I also recommend picking up a few hot sauces (Tapatio and Valentina are my personal favorites) and some cotija cheese to crumble on top (or a mild feta if you can't find it).
For an impressive-looking and semi-thematic dessert, you could make a coconut layer cake. This recent thread had a lot of ideas about that: https://food52.com/hotline/35737-i-m-looking-for-the-best-coconut-cake-recipe-non-traditional-coconut-cake-would-be-great-maybe-with
Another dessert idea is a tart filled with pastry cream (made with some coconut milk if you like) and topped with fresh fruit.

About that watermelon, mint and feta salad - it's a favorite of ours in the summer, especially when grilling. Here's how I make it, served on a large platter, not in a bowl:

Toss arugula plus a handful of mixed young lettuces with a simple lemon vinaigrette.

Put the arugula on a large serving plate; arrange bite sized pieces of watermelon on top; scatter good French feta and pickled red onions over the watermelon; sprinkle on finely chopped fresh oregano and mint; and then, drizzle on a bit more of the vinaigrette.

Do not toss. Make it even better with a handful of Marcona almonds at the last minute.

Hope this helps. ;o)

Antonia always give great and detailed advice. She is a tough act to follow. You seem to have it well planned so I'll just throw out some random ideas as I like to do taco parties too. I add mango and red onion to my pineapple salsa, also like sliced mango on the taco. I grill some thick avocado slices . Maybe grill up some firm tofu for the veg option. Good Luck!

Dessert suggestions:
Mexican brownies (with chilies) and/or tres leches ice cream
Margarita sorbet pie

Nice vegetarian option: make a black bean salad with drained black beans + charred corn (I use Trader Joe's frozen roasted corn) + lots of chopped cilantro + cherry tomatoes, quartered + a dressing made with frozen OJ concentrate (read John McPhee's long read in The New Yorker, or the book created from it, for why frozen concentrate tastes so good, without any additives whatsoever), lime juice, salt, pepper and olive oil. You could serve that as a side for the non-vegetarians as well, who will love you for doings so.
Also, grated cheese does yeoman's service on vegetarian tacos.
Put out a lime crema, too - sour cream + lime juice + salt, and slices of avocado for the tacos.
Others will have good ideas for the tortillas, but generally, all-corn tortillas do great if, after charring lightly on the griddle, you stack them in a deep casserole, putting a square of parchment between every 3 or 4 of them; tightly cover it and keep in a warm oven until ready to use. They can be refreshed in the microwave if necessary - stack, cover completely with a damp paper towel and cook for 45 seconds.
Finally, you absolutely must make several quarts of pickled red onions for your tacos! I've become a bit of a zealot about them. Recipe here: https://food52.com/recipes/29712-low-maintenance-fish-tacos See my comments to the recipe for variations. You may find the method for cooking the fish helpful for your party, too! I've made fish tacos using this recipe for salmon, mahi mahi, cod and halibut, all with great success. You must use a probe thermometer for best results, and remove from the oven any narrow portions that cook more quickly.
Sounds like a great party! ;o)