Aniston, Theroux appear 'giddy in love' on Hawaiian vacation

It's almost like they're on a honeymoon!

Jennifer Aniston and Justin Theroux are having the time of their lives in Hawaii. With Theroux's 40th birthday on Aug. 10, the couple touched down in paradise on Sunday on a trip that Aniston's "Wanderlust" costar originally planned with Heidi Bivens, his former longtime girlfriend.

So what have the sexy twosome been up to?

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On Monday evening, they shopped for gifts and checked out antique artifacts and jewelry for half an hour at the Lotus Gallery. "They were the only people in the store and seemed extremely relaxed and happy," a source says of the giggling pair. "Jennifer was hugging Justin from behind."

Finally, the pair made their choice -- a small Buddha tattoo -- and kissed at the register as a clerk wrapped it up.

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"When they left the store, Jennifer was massaging Justin's shoulders from behind, and he turned around and said something that made her laugh hysterically," the insider says. "They seemed giddy in love and genuinely happy together."

Later that night, wrapped Buddha statue in hand, they headed to Ben Stiller and wife Christine Taylor's estate for Taylor's 40th birthday bash.

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"Jennifer hung out with Christine but never got too far from Justin. They seemed to have a good time but didn't stay too long before they headed back to their house for the night," the source adds.

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On Tuesday, they left their rented house in a Jeep for a lunchtime treat -- buying a roadside coconut from a local woman, according to a source. Later in the day, Theroux went for a brisk half-hour jog through the fields surrounding their posh vacation digs. "Justin has a great body and working out is a big part of his life," an Aniston source tells Us.