A typical freestyle footbag
A footbag is the term for a small, round bag filled with dry grain (e.g. rice) or sand, which is kicked into the air as part of a competitive game or as a display of dexterity. "Hacky Sack" is the name of a brand of footbag popular in the 1970s (currently owned by Wham-O), which has since become a generic trademark. Footbag games are similar to earlier Asian shuttlecock sports such as jianzi. The most common game of footbag consists of two or more players standing in a circle and trying to keep the sack off the ground for as long as possible.
This is not an uncommon game for me because when I was a kid in the early 2000 I saw many other teens play this.