Zac Efron, Ashley Tisdale Party on the Beach
07/03/2011 AT 11:45 AM EDT
A beachside piggyback ride!
The shirtless hunk, 23, was spotted giving his High School Musical costar a friendly lift as they marked the VnC Cocktails and Heineken Light-sponsored milestone Saturday in Malibu, Calif.
"So happy zacary made it to my party!" Tisdale, who sported a striped Mara Hoffman bikini, Tweeted. "He's my brother/best friend!"
After frolicking in the sand – with buff and bronzed bods on display – Efron gave the birthday girl an encore piggyback ride as they headed back to a nearby friend's home to continue the festivities.