Zac Efron Post Rehab Reunion With 'Hairspray' Cast

10/11/2013 11:53 AM PDT

Zac Efron -- Post-Rehab Reunion With 'Hairspray' Cast

Zac Efron returned to the L.A. nightlife scene Thursday ... for the first time since news about his 2 stints rehab went public ... and it was all to celebrate HALLOWEEN!!

Efron appeared in great spirits as he hit up the Griffith Park Haunted Hayride with Julianne Hough and former "Hairspray" co-star Brittany Snow ... as well as director Adam Shankman.

TMZ broke the story -- the 25-year old actor completed 2 rounds of treatment earlier this year to combat the cocaine problem that took over his life. 

Julianne's brother Derek Hough snapped a pic with the group sharing hugs and smiles -- and posted it online afterward saying, "Great time last night with great friends."

You can't stop the beat.