How about to use Translate Instances node? I added the node after a Instance on Points node, and connected a Random Value node to Translation of the Translate Instance node.
For more details, please see following figure. 
Update
I found the other method that is to add Set Position node before Instance on Points node, and a Random Value node connects to Offset of the Set Position node.
I think this method is suited for many manipulations. For example, the method is easy to change point's Z-axis value according to input mesh's positions with sine function.
( Inspired by Youtube Video 1 and Youtube Video 2 )