I like Google Earth. The measure tool allows you to draw a circle of a defined radius...say 150 miles. Then it's easy to see what destinations are along the circle.
Google Maps doesn't have the same tool. If you know where you want to head, they do have a measure tool. Right click where you are and drag in the direction you plan to head until you get the preferred distance.
It is a good idea with either to run the directions function...particularly around mountains or bodies of water where straight line distances may be misleading...ie: if you have to divert 50 miles to the nearest mountain pass along a twisty road, it could be a lot longer drive than it appears.