I did about 10,000 swim practices in my life but amoong the few I actually remember best are the once or twice a year the coach let us screw around. My daughters’ high school has won multiple state championships. They work really hard (16 to 20 hours a week) but they also get 1 or 2 surprise coach takes the team out for food “workouts” a season. Its a great motivational tool.
For today, I suggest blowing off practice and taking the team out for ice cream, a late lunch/early dinner or doing something off the wall like having a poker tournament or craps game on the pool deck. They will love you.
If you are not back in the pool on Monday or can not find an alternate pool, hit the weight room and do some circuit training. If you keep the kids moving from station to station its a great workout.
Circuits. I do some dryland training with my HS team.
A fun one that we do is in pArtners. One does excercise the other does a lap(s). Usually a lot of core stuff. crunches/planks/push-ups/wall sits/ plus whatever you can dream up. They do the rep until their teammate returns. Can make a game out of it.
Chin-ups are great. If you have access to stretch cords/bands there are a ton of upper body stuff specific to strokes you can do.
Maybe run a pilates/yoga class for them. All of that stuff is beneficial.
Was just going to suggest ultimate frisbee myself. As klutzy as they normally are on land, even swimmers can manage to get a good and fun cardio workout during a game without too much risk of injury.
The club team my kids swim on does alot of ultimate frisbee early in the season. The kids love it. If you can get outside that will be fun and a good workout.