I think just to be funny gal you need to ask him when his work trip to Philly is and then get all excited and jump up and down and tell him you'll be there for work too! See what happens then.....
LOVE IT!!!
Or you can just, right before he leaves for it, tell him you "have a big surprise for him," and when he asks you what it is, tell him "you'll know soon enough -- for sure, he'll find out when he's on his trip." (of course, this is all just a smokescreen, natch).
