Mylar or Foil balloons are trending this season. You can inflate them with Helium or simply with air using a narrow valve and an automatic closing mechanism. To inflate it with Helium, it is important to have a thin valve and if you inflate it with air you must insert a narrow straw and then tie a knot.
If you want to inflate them with Helium, they will float for about 3 to 5 days. Enjoy them!