Tripp's wife gave Rufus a letter that was sent to Lily from Mr VdW, which I believe referred to the time they spent together when Lily was supposed to be looking after her sick mother. Rufus found out from Cece that Lily was not actually with her for as lon as she said she was. In the meantime, Rufus started flirting with another mother in the same building. Further compounding the ridiculousness of Doofus and Lily. She's a cheater, he's a sap.
Nate and Serena started something after she suffered the fallout from yet another bizarre relationship choice with the plastic looking Tripp. Nate told her he loved her, thereby destroying several years of storyline building in order to give Serena another boy to destroy while she flits along, being away with the fairies whilst trampling over people to get what she wants and managing to have everyone love her in the process.
Chuck saw a woman at his fathers grave who he thought might be his mother who was supposed to have died in childbirth.
Blair continued to be a disappointing social climber with no other ambitions or storyline, even the bulimia being forgotten.
Dan, Vanessa and Hilary Duff has a threesome where Dan realised that he was in love with the most annoying person on the planet, Vanessa. He told her and she ran away, which is a fitting response.
Jenny started to get involved with a drug dealer, and in episodes that you have not yet seen, seem to get into the most convoluted and ridiculous drug deals known to man.