Life’s a beach for Lindsay Lohan.

Just two days after turning 21 with a small shindig for family and friends at an oceanfront Malibu home, she was back seaside for the Fourth of July.

I’m told Lohan, along with her mom Dina, were among today’s guests at the nearby Polaroid House. “She was on a Promises pass with her sober companion—sober and chain-smoking,” a fellow guest reported.

Lohan’s former party pal Paris Hilton was also spotted at the house...though my spies did not see Lohan and Hilton interact