Here's a recent lesson plan I concocted for working on forehands. The serve may be 'the most important shot of the game', but forehands are the ones we hit the most (80% of shots by some estimates). So we spend a lot of time on forehands on my courts.
Spread players out around baseline based on how large of a group you have. One player, or the coach, is in front of the group. Front player leads with 10 forehand shadow swings. All others copy him/her. Each player takes their turn as leader. In a large group you will have hit 50 or more shadow swings by end of this warm-up. Make sure players begin with and return to ready position; are taking the racquet back properly; have a point of contact out in front; and a good quality follow through. I also ask my players to track the ball with their non-dominant hand. Coach circulates among players during warm-up, fine-tuning swings as needed.
Two players are at service line (ad/deuce). Place two spots for each player: one where the player stands/begins and another at where forehand should bounce. This will be at 1-2 o'clock for righties and 10-11 o'clock for lefties, about racquet length distance away from player. Helpful if the bounce target spot is the same color for both players and different from the beginning spot, so you can remind the tossing players to aim for the 'red' spot or whatever color.
All other players are across net at service line. If you have a large group, divide in half. These players are taking turns tossing underhand balls toward bounce target of player straight ahead. Toss must be good quality with step from opposite foot, similar motion they will be using when hitting. Object is for tossing players to hit bounce target before hitting player hits five forehands in. Ball hitting bounce target is instant win for tossing team. Once a win is achieved, two new players rotate over to hitting positions.
Continue until all players have had a turn at hitting. Repeat, this time tossing cross court. Repeat both straight ahead and cross court toss, this time moving hitting players back to baseline.
I continue this activity until we are about halfway through our lesson time. Then we warm up serves, and finish with some live ball points.
Note: some players, when on tossing side, are determined not to let the hitting players 'win', and toss some really ugly stuff. Reinforce their goal is to hit the target, not to hurl fastballs. Insisting on good quality form for underhand tosses goes a long way toward eliminating near-impossible feeds, but still - keep an eye on this.