Mark a line on the wall. Take a few steps back. Repeatedly volley, or bump, the ball above this line for as long as you can! Have someone time you, or simply count the number of repetitions you get in. The goal is to keep it up and above the line.
Tape, chalk or other method to mark the wall (make sure you ask parents permission first if doing this in the house).
A volleyball, or any other type of light bouncy ball or even a balloon.