I don't play contact sports; but I do play badminton at a very competent level.
Restarted games 3 months post donation (had some minor complications which delayed the restart).
Currently I'm 5 months out, and I feel that I've got the rhythm back - say at least 90% of it. (I have slightly adjusted my game play by being moderately offensive, as against highly offensive earlier; to adjust mostly for the stamina and strength part).
Every day, however, I go back home with some stiffness in the stitch area (probably due to overusing the muscles due to jumps and smashes); but the good thing is that even the stiffness has come down drastically. (It helps to take a break of a day or two in a week to recuperate).
Meanwhile, I believe the advice to not engage in contact sports is mostly aimed at being cautious in games like American Football, Rugby, Boxing, etc. than games like Soccer, BasketBall, etc. (I admit it does have some contact involved unlike tennis/badminton).
Advices from my personal experience (although I'm still on the learning curve):
1. Take more care of your stitches and area surrounding it than the other kidney
2. Take a break of a couple of days in a week (it really helps the stitch area to calm down)
3. Water, water, and water.
Best Wishes.