My son Lincoln came up with a great game on vacation.  Wherever you are, you pretend there are minibosses all around you that you have to defeat, but they are actually inanimate objects like trash cans, signs, and chairs.

You decide how to defeat each one, things like:
*Roll the ball under the table to hit the garbage can
*Bounce the ball and catch it 10 times in a row in front of the sign to defeat the sign (add a spin for extra difficulty)
*Toss the ball and try to get the ball to land on the chair
*Throw the ball in the air and catch it 3 times in a row without it hitting the ground

The possibilities are endless and only require a single tennis ball and a lot of imagination.  Great game, Lincoln!