Kari,
You could also have a Pinata. I am not sure if you need to plan any activites to go along with the banquet, but if you do, a Pinata would be a great addition.
I also agree with the taco bar suggestion. Too many other dishes require lots of prep and cooking time. You could add a selection of different shells for the taco bar such as; soft tortillas, hard taco shells, tostadas, nacho chips, etc. For the fillings you can have rice, beans (pinto/refried, black), ground beef, etc.
There is an online website where they have lots of theme decorations for parties:
www.partyadventure.com
Red, white, and green balloons (to represent the Mexican Flag) are a must have, and their cheap by the dozen!
Oh, and don't forget to have some Mexican music playing during the Fiesta! Ole!
Have fun!
7mozz