You could make a function to generate random numbers to use for locations or use the plugin worldguard (I think?) to make regions and work with it

 

Code (Java):

1. every 10 minutes:
2.  
3.     set {_x} to random number between 5 and 10
4.     set {_y} to 60
5.     set {_z} to random number between 0 and 10
6.     set {_world} to world "world"
7.    
8.     set {_location} to location({_x}, {_y}, {_z}, {_world})
9.    
10.     spawn zombie at {_location}