Workouts in Open Water - Any fun things to do?

All of the swimming pools in Germany are closed and will likely remain closed for a while.  Due to this, I have been swimming in (very cold) lakes and ponds.  I am looking for some fun workouts to do while swimming in open water.  

Any advice for fun training games? 

I've tried the following: 

  • 25 strokes FAST + 25 strokes EZ 
  • Breath control (1 stroke + 1 breath followed by 2 strokes + 1 breath, 3 strokes + 1 breath, and so on until you have reached your max)
  • 20 strokes ea stroke to simulate a 100IM

Thanks for the ideas!

Coach Michi