Jennifer Garner jeans sneakers plaid shirt William Sonoma

Jennifer Garner looks lovely dressed-down, but its impossible to hide her movie star looks!

We caught up with the plaid-clad mamma while she hit up Williams-Sonoma for some housewear goodies.

The Arthur actress celebrated her 39th birthday Sunday with a super low-key, outdoor barbecue at the Pacific Palisades estate she shares with hubby Ben Affleck. Some 30 guests attended the party, including Matt Damon and his ever-growing fam.

Before the party, Garner was treated to an extensive spa treatment at the ultra-expensive Peninsula hotel in Beverly Hills -- the perfect gift for busy moms!