wb,
If you are in a position where you will be practicing alone then I would suggest wall ball. Use a tennis ball or rubber baseball and bounce it off a wall back at yourself. Make it more challenging as you get better. Imagine the plays you will be making and go through the motions.
Anything else you can think of that will improve hand eye coordination, glove to hand tranfer and overall quickness can't hurt.
Take in some games and pay special attention to what goes on in the infield. How they set up, where they catch the ball and how each position player plays his spot.
As far as hitting, I would get a tee and a bunch of wiffle balls and swing every time you have the chance. Make a game out of it to see how changes in your swing and your stance will change your results.
Ask for advice whenever you have questions about particular parts of the swing and fielding.
Good luck and have a great summer!!