Some ideas for inputs and effects:

• a wifi sensor that reacts to the number of people present. Perhaps the number of musical tracks enabled is based on the number of wifi stations present.
• floor pads that can be used to determine the amount of movement present. I really like this idea because they can be concealed and the purpose is not at all obvious.
• range sensors to see how close people are to various points
• various wheels, levers and buttons. (ie. a button that enables a track, but that track may only add a sound effect every 10 seconds.)
• a light sensor that can change the mood of the music from sunny daytime to ambient night time.
• connection to a weather report. Play gloomy music if it is cloudy. Increase the tempo if it is cold. etc.
• anemometer can play a wind-chimes track. Louder for more wind.

Of course, there will be a requisite number of blinky lights and twirly things, but all effects will be controlled by a particular midi track, and interactive will only be to apply a midi effect, not directly control the output. So, a blinking light may blink according the a rhythm of track 3, and a pitch-bend effect may change the colour of the light, while the tempo change will impact the speed of the blinking.