W.C.
Those are fabulous ideas. Do they have a theater department? What about back drops from plays? Is there an outdoor scene that you could place on the stage? What about a park or Paris? Balloons are fantastic. I love this design for either of these ideas. http://anoccasionstation.net/gallery1.htm
Or the trees that you described in mass would create a fantastic ambiance too. You could hang red and pink Christmas balls from the branches too. Wal-Mart has a ton on clearance still. I think with red and pink with pink and white tulle would look very Valentiney. Is that a word?
And here are some good balloon designs depending upon how much you want to use http://anoccasionstation.net/gallery2.htm Imagine these with huge balloons and really tall. Very dramatic.
You could hang a wire or string from wall to wall and hang paper lanterns from the wire.
http://www.partybuddies.com.au/BalloonDesign/pink_dolphin...
http://www.asiaballoon.com/ps/images/illu-pillars.jpg
http://www.apartyhearty.com/services.htm
I think that choosing one of these ideas and doing a bunch of them would give the best results. Set the stage on the stage and carry out through the rest of the gym with as many whatever as the budget/time allows.