This simple array will attract mobs to it from a fair distance : 10 blocks on both axis. This array can be made by 2 pieces of rotten flesh into a blank array:
This array works great for mob farms if you don't want to have running water, play around with how close or far you need this array to be, putting it behind any kind of mob killing machine should work well. However, even if the mobs run toward the Array, they still keep a sort of survival instinct which prevents them from falling in any deep, painful and/or deadly hole on their way. However, if the Array is placed above a hole and if there is no gap between it and the last solid Block, the mobs won’t notice it and fall in.