Solar System PBL

I have a solar system PBL planned for next week and would like to take the students to the WVU planetarium as part of the lesson. I have planned a trip there for next Thursday, February 28^th. PLEASE try to keep it a secret. I am basing the whole unit off of their love of The Magic School Bus and kind of want the trip to be like something Ms. Frizzle would take them on. I sent home a permission form. It is folded and stapled with a pink "please sign and return" sticker. I have quite a few students in my class that would be able to read the note and tell everyone in the class where we're going. I know it might be silly to keep it a secret, but imagine the look on their faces when they find out next week.