Jennifer Aniston and Justin Theroux
08/03/2011 at 03:00 PM EDT
and boyfriend Justin Theroux
have taken their romance to the islands, enjoying a relaxing getaway on Kauai, Hawaii, since July 31.
On Monday, the couple, who are planning to move in together
, helped actor Ben Stiller and his wife Christine Taylor celebrate Christine's 40th birthday at an oceanfront villa. Before attending the birthday party, the couple stopped by the Lotus Gallery of Fine Art to pick up a gift.
"They seemed to have a great time," an eyewitness says of the smiley Aniston, 42, and Theroux, who turns 40 on Aug. 10.
Aniston and Theroux went sightseeing across the island Tuesday – and made a stop for tropical juice and fruit at Banana Joe's Fruit Stand.
"They are acting like regular tourists," says a source. "They have been touring the island in a rented Jeep ... Justin is beyond sweet and attentive to her."
The globetrotting couple
are also doing their best to stay swimsuit-ready on vacation.
"They have been jogging by their villa and they are both in great shape," adds the eyewitness.
With reporting by PERNILLA CEDENHEIM