They are two kinds of minigames : 

• Parkour and Maze
• the actual Minigames

The Parkour and Maze reward a small amount of Nano, currently 0.001 Nano for their completions.

The Minigames allows you to bet Nano with your friends on who's the winner will be.

Where can I find those games ? 

Simply head over the spawn and you'll find stairs next to it leading to the server nexus that contains portals to teleport you either to the Parkour and Maze or the Minigames.

How to bet on Minigames ? 

Simple join a minigame and the use the following command :  /games bet *amount* to bet on who's the winner will be.

If you're the only gambler in the game, you'll automatically get refunded.