A great idea when turning in some hours on the practice courts is to play an occasional tiebreak with your practice pal. That can help big time with putting those things we've practiced to work in a competitive setting and more quickly learning to trust them. It's also great for keeping practice grinds and drilling more deliberate and focused. When the breaker is over and that mindset is still fresh in our heads, that mindset can be applied to practicing better placement, consistence, spin variety, etc. that we need to use through point play.
I like to sometimes hit cross-court forehand to forehand or backhand to backhand instead of just straight down the middle. It's often surprising to me how demanding that can be even when not playing points. You can also try hitting baseline rallies using only forehands or backhands. This can shift your footwork into extremely high gear. Try to hit maybe twenty balls in a row or some sort of goal like that.
Spend some time at the net and let your hitting partner pound on you (and return the favor). Those reaction volleys become more ingrained the more we practice them. If you have a bucket of balls, you can also drill each other by having the feeder pepper the net player with high frequency feeds that aren't too fast. The idea there is for the volleyer to get the strings to the ball and instantly recover for the next shot. The feeder can set up around the service line and when I feed, I like to hold my racquet up on the throat - pop, pop, pop.
Hit overheads. More than three or four. I'm thinking maybe two dozen or so.
If you want to really burn it up, you can do baseline rallies where one of you hits all cross-court while the other hits all down the line. Maybe switch off between this and a mode of more static hitting like down the line to each other in an alley while you gulp some oxygen.
Returns of serve!!! The feeder can hit serves from only a step or so back from the service line and challenge the returner with all sorts of pace, placement, and variety. It can help for the feeder to maybe say whether he/she is sending a first or second serve. This close-in feeding can let the server hit a lot of balls without getting too tired too fast. I've had a ton of success using this with the high school teams I coach.
