Word Chain
Play an icebreaker like "Word Chain" to get the party going. Sit everyone around in a circle and start the game with a word that has to do with either the birthday person or the theme of birthdays. The next person in the circle has to restate your word and add another word within that same vein. This process continues around the circle as people get disqualified if they forget a word in the chain. The game ends when one person--the winner--remains in the circle, having never broken the word chain.
Siamese Gift Wrap
If your adult party guests relish in competitive games, try out this birthday game from Party Game Ideas. Set aside a few gift boxes and some wrapping paper. When it comes time to play, divide the guests into pairs and hand each team a box, a spool of wrapping paper, some tape and a pair of scissors. On the count of three, each person has to work with his partner to wrap the box while using only one hand. The first team to wrap the box wins the game.
Message under a Plate
Write strange phrases under each guest's paper plate before you serve the birthday cake. Such phrases include "I like to embroider at night" and "Monkeys like bananas." To start the game, each guest looks on the bottom of their plates to see his phrase. The guests then try to casually inject their phrases into the conversation without anyone figuring out that the phrase was part of the game. If a guest does get accused by another player, he gets disqualified. Those who successfully used their phrases in the conversation get prizes or bragging rights (See Reference 3).