Charlize Theron and Sean Penn rang in the New Year together in Hawaii, where they "seemed happy," an onlooker tells Us Credit: HACO WAHS/AKM-GSI

Just friends? Charlize Theron and Sean Penn set tongues wagging this week after they were spotted ringing in the New Year together in Hawaii.

Theron, 38, and Penn, 53, were photographed on the beach together with another pal on Monday, Dec. 30, in Oahu's Waikiki area. Two days later, on Jan. 1, they enjoyed breakfast with friends at the Manoa Surfrider, a source tells Us Weekly.

"They seemed happy to be spending some time together, very comfortable," the onlooker said of the Oscar winners. "They were both pretty relaxed and dressed down."

The Gangsters Squad actor, who has a home in Hawaii, was also photographed catching some waves on his surfboard. Theron, meanwhile, showed off her impressive bikini body while enjoying a solo stroll along the shore. Both returned to L.A. on Wednesday but were careful to exit the flight separately.

Penn has been linked to several high-profile stars in recent years, including Scarlett Johansson. (He and ex-wife Robin Wright split in 2010.) Theron's most public relationship was with Stuart Townsend, whom she dated for nine years until January 2010.