The hover disc and hyper disc (another brand) is an outdoor toy. It is a rather large, about three feet across, inflatable toy that acts like a super Frisbee. It can do the things a Frisbee can do, but is versatile enough to do so much more. It can be bounced from the ground to great heights, it can be floated through the air, and most important, it can provide hours of fun.
The hover disc is an inexpensive toy, and can be found for well below twenty dollars. Some hyper discs price below ten dollars. If you are looking for economy you should avoid the licensed images that some have. Those popular characters may be nice, but they add to the cost. If this is to be used for just a few days you might opt for the lower priced ones.
Since inflatables are subject to damage when striking pointed objects like fences and branches, and can be lost high in trees, having a spare or two is not a bad idea. It also gives multiple children a chance to all be included, even if one child takes one hover disc over.
Of course helium can make it lighter, and make it float better. But, I suggest you start with air, which should be good enough for hours of fun.