With many years of coaching youth, high school, womens, and college fastpitch softball, I found one drill that fits every level of play. It developes both the INFIELD mechanics, OUTFIELD mechanics, and finally RELAYS to HOME.
I divide the team into 2 groups. One group lines up behind third-base. The other group lines up in deep right-center. I have an assistant coach acting as my catcher as I hit hard grounders to the first in line at third-base. She fields the ball and quickly makes a sharp throw to home-plate. I then hit a deep fly ball to the first in line at right-center. As this player sprints to catch the ball, the fielder that was at third-base sprints to a relay position for home-plate. The outfielder hits the now relay fielder who should be properly positioned to receive and relay the throw to home-plate.
The cycle repeats as the infielder that was the relay joins the rear of the line of outfielders. The outfielder that caught and threw the ball to the relay then joins the rear of the line of infielders. This cycle should be repeated as to correctly get everyone to errorlessly field, catch, throw, and relay through an entire cycle.