Hi!
(My $0.019997)
I will have a similar problem, involving an extended vacation in Europe, in about 5 hours [its 12:30 AM local as of writing

]. I really honestly don't know what to do myself. I have 'experience' in being unable to practice for extended periods of time. What I notice is, firstly, is that returning to the piano involves a great emotional and musical enlightenment, the realization and triumph of the various themes circling through one's thoughts for months. Perhaps think through the pieces in your head, both in sound/notes and in the physical act of playing, like Chopinetta said. Tapping fingers works best with fast pieces and longer surfaces, not necessarily 'flat'. Avoid playing in air, its less helpful for dexterity and terrifies civilians. I agree with Mr. Arrau's (tangentially which of his recordings are good? I have heard enthusiastic comments about his recordings but I've only heard [I think] his Chopin etudes, which are in fact beautifully done) teacher, it does unstale one's playing, although I would add that absense from piano would also disrupt one's practice regimes and would be preferable in so called 'interim' time such as summer. When returning, start first with something you have mastered completely and that you know very well, to regain the flexibility and strength of the fingers, from which all else will follow. Its very difficult to forget a piece, I can sight read difficult pieces I haven't looked at in years, third time through impeccably under performance tempo; the musical subtleties come back with great facility, as they are memorized less peripherally than things like grammar rules. As your case is a school trip, you really will be without any practicing. In my case, it being a vacation, under what conditions can one practice in foreign countries, in a serious way? What about university pracitice rooms? Do European universities have practice rooms like here? I am taking up waaaaaaaaaay too much space.
Good luck!