Not sure what you mean by a step down filter, you can get step down rings which will allow a bigger filter to be attached to a lens smaller than it was designed for.
You can get macro magnification filters which will screw onto your lens essentialy they act like a magnifying glass or as you have correctly identified you can get extension tubes which fit between the lens and the camera.
The main disadvantage with extension tubes is with cheaper models you can loose autofocus, you will get less light hitting the sensor so this needs to be considered also a slight loss of DOF.
Filters can be a bit tricky to use and also you can get distortion having said that you can still get good results.
Given that you have an 18MP sensor you do have a lot of room for manouvre with cropping in PP to enlarge your subject of course.
I have the 100mm EF Macro and know what you mean sometimes I want to get closer and would love one of
these but really cannot justify the cost plus they are a pig to use.