We did a Cinco de Mayo theme a couple years ago. We served tacos/taco salads and decorated with red, green, yellow, etc. We had our parents donate all the supplies to make the tacos(coordinated with the room parents) and we also got donations of desserts and beverages.
All three companies have great fiesta items. Shopping with shindigz can be quite expensive.
Does your PTO ever ask parents to donate food for appreciation luncheons? We ask parents to donate side dishes and desserts, and the PTO pays for everything else.
Just have great food and great decorations and you won't go wrong! Have fun!