With 60 people, 30 minutes is barely enough time to go outside and come back in, even without doing anything at either place. So you might be limited to doing a demonstration instead of participatory project.
It might not be worthwhile buying $80 worth of helium just so people can see what a balloon looks like inflated, although a five foot inflated balloon is a very good way to make people remember something. Ballooning and kiting are so weather dependent that either activity could be less than successful during any assigned 30 minutes. Both require an open, safe area, and kiting requires a big, open, safe area and some practice. In 30 minutes you might have to choose between watching a kite or balloon and looking at the aerial photos taken from one. Unless you have a few well trained assistants, it's hard to do both.
So I like your original idea of using the smart phone spectrometer. I takes longer than 30 minutes for a first-timer to make one, but you could make 10 or 20 of them in advance in about a day. If you do a good job building them, it will take people about 10 minutes to make adjustments for their phones and figure out how to take good photos (exposure, zooming, aiming, etc). Or a similar time to figure out how to connect to Spectral Workbench to see a spectrogram of the photos or video they take.
If you have some accessible fluorescent lamps that people can walk right up to and point their phones at, you can get the compelling spectrum of distinct emission lines. If they can connect to Spectral Workbench, people can use that image to calibrate the spectrometer. By the time they figure out how to do that there will not be any time left to take a spectrum of something else and use spectroscopy to learn something new. But they can take their spectrometers home and do that.
The plans to make them are here: https://publiclab.org/sites/default/files/8.5x11mini-spec3.8.pdf. You can hand these out so people who don't take home an assembled one get something. It looks like the store is out of the $10 kits. Jeff might have plans for a new version.
If you want to do a demonstration, you can use a calibrated spectrometer to capture the spectrum of something biological like chlorophyll. Sunlight or incandescent light passing through a suspension of blended fresh leaves (spinach or algae) in alcohol or even water will have a lot of the blue and/or red end of the spectrum absorbed (notes). This is usually easy to see and easy to explain with science.