Really, any
on-gain card.
Embassy comes to mind, definitely. I can think of a few plausible cases for
Inn if you're feeling lucky in terms of shuffling.
Ill-Gotten Gains if you value your friendships
.
Opening 5/2, if you went first, Feast would allow you to defer a strategy decision until after you've seen your opponent's first couple of turns. Similarly, it could mask your strategy if you were setting something up which didn't have much use for its $5 component in the first few turns.
I think opening Feast/Copper for 5/2 is underrated. You lull your opponent into a false sense of confidence, then you build up your engine while he's busy explaining why your opening sucked. Dominion is, after all, a largely psychological game.