1) Charity auction -- who wouldn't pay tons of galleons for a couple of evenings with Harry Potter? Harry's very reluctant to be auctioned off, but does it for the charity. He convinces snape to save him from the crazy fangirls by bribing him and giving him his own money for the bidding. Of course, since Harry knows Snape doesn't particularly like him, he assumes Snape won't claim the evenings together that he's "won", and is naturally very surprised when Snape does. (I think this would be particularly lovely with a cautious, IC snape, and tension developing over their time spent together).
2) Cross-dressing!Harry -- Harry is dared to wear a girl's uniform for a day's worth of classes and discovers he rather likes the feel of a skirt beneath his robes. Snape suspects harry is up to something over the following weeks (which he is, occasionally wearing girl's clothing, hidden) and finally confronts harry only to discover a rather ... lovely surprise. (Ideally takes place when students have returned or are repeating their last year after the war).