“Gossip Girl” (8 p.m., The CW): Michelle Trachtenberg, an old friend from “Buffy

“Gossip Girl” (8 p.m., The CW): Michelle Trachtenberg, an old friend from “Buffy the Vampire Slayer,” resurfaces on “Gossip Girl” tonight. She plays Serena’s former partner in crime who is back in town to stir up more trouble.

“Sweat Equity” (8:30 p.m., DIY): The more you sweat, the more you will see the green.

“House” (9 p.m., Fox): “House” finally returns with the show’s first episode since February. It has our cranky doctor questioning the disposition of an ER patient, who just seems too happy to be true. House might want to try it himself sometime.