There are SO many things that each athlete can do on their own to work on their volleyball skills. Here's a short list:
Pass to yourself while standing
Pass up against a wall (make sure this is okay with your parents!)
Set to yourself while standing or sitting.
Set up against a wall.
Practice your overhand serving toss/lift up against a wall to get it consistent
practice trapping your overhand serve toss up against a wall to learn where to make contact on a hit/serve.
Practice serving (under and overhand) against a wall or in your yard.
The following YouTube videos have some more great ideas for work that you can do on your own to improve your skills.