Summer is a bit hot for people to wear full fledge costumes. So if your birthday happens to be in the summer and you are set on a costume party, try to do a theme that involves light garments.
Hawaiian Themed Party
The best one is a
Hawaiian themed party.
It is even a better theme than many others because there are lots of
easy decorations available (tropical flowers, flower garlands,
seashells, etc),
drinks with tropical fruits and little umbrellas, and you can play
Hawaiian music. If you want to go that extra mile, you can also serve
Hawaiian food.
As they arrive, greet each guest with a
Hawaiian lei in the traditional Hawaiian style.
Organise Hula (grass skirt) dancing competitions, Limbo dance, and Hula Hoop contest.
Roman/Greek Costume Party
A
Greek or Roman costume party is another good one for the summer because togas are made of white, light fabric.
The guys can go as a god with a white tunic, a Roman Emperor or Caesar, a Gladiator with just a skirt without shirt, or a Spartan.
Women can dress up like goddess with the white or gold tunics or Greek/Roman nobility with the same kind of dress.
For decoration use plates with food, lots of cushions (seating can be on the floor in cushions or on low couches and lounge chairs. If your budget allows it you can also
hire some props such as statues and columns.
The main drink must be wine served from an amphora. For food use seafood and Mediterranean salads, couscous and olives.