Volley-to-volley drills are good for improving reaction times generally,
but here's a good practice to help you improve your reaction times on
return of serve:
Practise returning a friend's serve, but simulate a much faster serve
by getting him to serve from the service line instead of the baseline.
He'll need to place the ball in front of him and use a lot of wrist snap
to avoid hitting the ball long, but he should get the knack after a few
attempts. As for you - you may need to adopt a neutral grip (halfway between
your forehand and backhand grips), you may have to adjust your waiting
position and you may have to use an abbreviated swing. Don't play the
point out. Return half a dozen from the deuce court, then half a dozen
from the ad court, then the two of you can change roles. After a while,
you can make it competitive by counting how many of the 12 returns land
in court and your friend can try and beat your score.
The abbreviated swing is achieved by placing your elbows close in to
the side of your hips instead of out in front of you. That way you'll
shorten the radius of your backswing, making it faster.