I bought my son a couple UFOs for Christmas. Basically 10’ long black cylindrical garbage bags. You fill them up with cold air, seal them, and then put them out in the sun.
They expand it the heat and begin to rise. In five minutes they float. In ten minutes you really have to hold them down or they take off.
They would continue to rise all day as long as they are in the sun!
F lying
B ag
I nspectors