Please post the age group you will be coaching. There are as many different plans and drills as their are coaches but they are usually designed to age group. Though one thats always fun for any age is to get them to practice pass and set by trying to make baskets in a basket ball net.

I will be coaching jr high & high school girls

Since you were an asst. last year start with some of the drills they already know. Here is a site that has a lot of drills to choose from:

http://www.volleyball.com/drill.aspx

Definitely make up an agenda for each practice, and make sure you switch things up so practice doesn't get repetitive and boring. When you make up your agenda have every minute accounted for like so:

2:00 Stretching 5mins
2:05 Warm up bounce circle 5 min
2:10 Partner pass drill 15min
2:25 Partner set drill 10min
2:35 Serve instruction 5min
2:40 Serve practice 10min
2:50 break 5min
2:55 defense instruction 15min

and so on, at this age you don't want them to have down time because they will clown around. And you don't want them to see you trying to figure out what to do next. So the agenda prevents this. Don't worry about scheduling too much, you can always skip a drill if one runs too long.

I find the best drills, and the ones they find most enjoyable, will get them running around the court. It builds stamina and teaches them to move, which can be difficult to get across at sometimes at this age. I schedule the boring rote drills right at the beginning of practice, and them get them into some fun stuff. Two of their favorites are:

The pit: One or two players are given a small area to stand in, (the pit). All the other players spread out around the court. Then the coaches take a cart of balls and start throwing them one at a time to the players in rapid succession. The object is that no ball touch the floor without being passed. The other players shag the balls. After 2 min or so many balls, the players switch off until all have had a chance in the pit. Don't let players throw the balls as that can get out of hand. This can be modified by adding a target for them to pass to. Teaches movement and communication.

Serving race: Separate players into teams of 3 ea. across the back line. Each player has a ball one at a time they have to make a good serve then retrieve their own ball from the other side of the court. The next player cannot serve until the one in front has made a good serve. The most good serves in 5 min wins. The limit of 3 forces the players to run to retrieve their ball and get back in line to make the next serve before the previous player is done. Teaches serving accuracy and movement.

I try to make most of the drills up to include movement, as this is one of the most lacking characteristics at this age level. Make the drills into games of competition and the kids will have more fun, but make the games about accuracy so they don't get sloppy trying to go too fast.

Good skill to you and have fun.

Thanks for the advice.
What is a bounce circle?