My top three training aides are simple and very effective.
1) A shoe box to work on returning the club square to the flat part on the back to learn a square clubface, lengthwise to work on swing path (not too much outside in or too much inside out).
2) A can of foot spray to spray on the face of a club to see where impact was made.
3) A marking pen to make a mark on the mat for an image of where the club should brush the grass.