There are some simple things that you can do to make it special, and that will impress all of your friends:
Finding some fun place settings for cheap, or even making your own, add an even more intimate touch to the whole evening. Simple things like napkin holders— take old paper towel rolls, cut into sections, and wrap them in decorative papers or fabrics. Add old buttons or other embellishments to make a cute little accent to your paper or cloth napkins.
Painting your own margarita glasses will make the night more personal, and let your friends feel special because they have their own glass, that is one of a kind.
Margarita glasses can come in all forms and prices, in both plastic and glass, depending on your budget. Simple acrylic paint from stores like Michaels or Joann's would work fine on either surface. You might even have some kind of paint around the house to use (make sure it is not toxic since your friends mouths are going on the glasses). A good place to look for margarita glasses would be a party supply store, they would carry quite an array of different styles of glasses.
For the summer, fun floral patterns would be cute on the glasses, maybe a different flower for each friend. You could even put your friend's name on the glass so they know which one is theirs.
Of course you will need something to put in said Margarita glasses. There are tons of recipes for things like daiquiris and margaritas. One of my favorites is the skinny girl margarita, its delicious and low (enough) on calories! Here's the recipe:
2 oz of clear premium tequila
.5oz of orange liquor
juice of one fresh lime
add ice, just mix and enjoy!
As for food: light, fresh foods are the best on a warm summers night. Summer salads and light finger foods would be perfect for a casual, night between friends. Pick foods with a lot of color!
Other things that would be important for a perfect night, are mood lighting and other decorations. The more random the better, it gives an eclectic homey feel to the night. Candles, Tiki torches, rope lights—deck out, well... your deck or patio in the lights. Also find the prettiest garden in the neighborhood and ask your neighbor for some flowers, or purchase a wild flower bouquet to add to the table. Wild flowers tend to be more casual but beautiful, and perfect for a night such as this. Have your food on large serving dishes, or bowls, those will help fill the table up with more color!
This page has a ton of fun, light summer recipes that are nutritious and fun for outside eating. All are simple to prepare and sound delicious. AND they have a lot of color to add some pop to your table. Here's to a great Girls' Night In!