Hi Bobby,
There are no wrong or right answers to your question. That's why you've been given several differing answers. Any speed film will record an image, but they'll be very different styles of image.
So you should really ask yourself a couple of questions.
1) What is your intended subject? The balloons? People in the crowds at the event?
2) What results do you want to acheive? Are you hand-holding the camera? In that case, the faster the better. You'll be able to hand-hold effectively at 1/30 at f2.8 or narrower aperture at about 1000 ISO in artificial light. You'll have limited depth of field and grainy images.
Or maybe you'd like to experiment with longer exposures and have less grain. Do you intend to use a tripod, cable-release and/or self-timer? Lens cap? You could use a 100 or slower film with multi-second exposures. Moving people will vanish, the balloons will blur if there's any wind at all. You can use a narrow-ish aperture, say f11, to keep everything focussed. Images made with this technique can look surreal. This is the option i'd choose.
3) Will you be close to the balloons, or far away? If you're right up close, you'll be able to use a faster shutter speed / narrower aperture. You might even be able to hand-hold a 100 ISO film and obtain sharp results. if you're miles away, you'll get less light coming through the lens.
As an example,
this link gives an idea of what you might expect to achieve from a hand-held camera without flash at around 200 ISO (rough guess that...)
As you see, it's difficult for folks here to answer your questions when we don't know your intentions. IMO, there's no 'best ISO' for anything really - you just need to decide what results you want before anyone can really advise you.
If all else fails, use f8 and be there!
Hope that helps,
kevs