A beautiful necklace
Right before their wedding and before Howard headed off into space, the aerospace engineer gave Bernadette a star-shaped pendant necklace on "The Launch Acceleration" (Season 5, Episode 23).
However, she'd have to give it back so he could take it with him to the International Space Station. Then, upon his return, he'd regift the bauble, which would posses an even greater meaning because the star would've come directly from space. Astronomy never sounded so sweet.