IMO, the practice period most likely to have "down time" is (on field)BP. So we do a number of things to insure that everyone is working to get better, like:
1)Dividing our players into specific groups, like "A" infielders, "B" infielders, "A" outfielders, etc. You then can set the rotation where you always have infielders on the infield, outfielders in the outfield and so forth. One round of our BP might go like this; "A" IF hitting on field, "B" infield and "A" outfield on defense, "B" outfield running bases, and Group "C" (catchers and extras") in cages. It takes extra prep time, but we designate ahead of time on a BP organization sheet where each person will play defense during BP.
2)On the BP organization sheet mentioned above we will designate "hot" outfielders. These guys play the ball live off the bat. If a ball is hit foul or gets by them somehow the other OF (behind them) will retrieve it and the hot OF gets ready for action. Putting cones down in a shallow arc and requiring the OF to field balls on the ground in front of the cones increases effort.
3)We also have a manager (we have only two coaches, too) keep a fielding chart during BP and the coach not throwing keeps a stop watch to time the IF's on ground balls. If the IF exceeds the stopwatch time you set, you rule it a hit or an error. We stole this idea after hearing the coaches from Vanderbilt talk about it at the ABCA clinic a few yrs ago. Players get credit for put-outs, assists, errors, and "ESPN" plays daily, and we keep their fielding percentages. Posting practice fielding %'s is a motivator that keeps them hustling.
4)We also often use BP's (like the Six Outs BP Andy Lopez explains in his book "Coaching Baseball Successfully") that require every player to give maximum effort in competition against other groups.
I would urge you to seek out some volunteers from your community to increase your coaching staff and recruit managers from your school. Our managers our younger players who love baseball and are going to be playing on our team in the next few years. They provide help while also learning your system a few years early!