In the first scene after the opening credits, Summer and Marissa are sitting by the pool. This exact same scene, albeit with different dialog, occurred in the second season premiere right after the credits.
The bus Trey takes is headed for Las Vegas. During last season's finale, the plan was for Trey and Jess to leave Newport to go to Vegas.
While discussing addiction, Kirsten's friend Charlotte describes raiding a pantry late at night in search of alcohol, only to drink a full bottle of vanilla extract. This exact scenario takes place in the second-season episode of Family Ties ("Say Uncle") in which Alex discovers his Uncle Ned, played by Tom Hanks, is an alcoholic.