You could of course play mexican/south american music. You could even hire a mariachi band to play for a while.
For props I would definitely add some maracas, a sarapi, the sombrero, etc.
You might want to set up a tequilla tasting, similar to a wine tasting. This could be expensive, depending on the number of guests. You might also have a little brochure or card that lists the brief history of tequilla and list the brands you have to taste.
Each guest could get a shot glass or margarita glass as a favor.
If the party will be at home or in the back yard, you can string chili pepper lights around. (cheesy but cute)
If you want real mexican food, check into having your favorite local authentic restaurant cater.
Here is the link to the FoodNetwork web site all about fiestas if you want to go for a more upscale feel: Cinco de Mayo Recipes, Margaritas, Fiesta Tips: Holidays : Party Ideas: Food Network Can you say YUM to Kaluha Flan!