The other day when I was at work my son was 'standing' on my bed, slipped and sprained his neck. He visited the doctor with his head at quite an angle, or relative to his right shoulder, his head made an acute angle. See Monica's blog for more details. (My wife is a great storyteller.)
His neck is getting better. Notice how it is nearly orthogonal in the picture to the left. He is wearing his helmet that he just accessorized with a 'laser' and Wall-E pictures. (No, he had not been bicycling, but he is all about safety.)
An enjoyable coordinate system that I encountered again the other day was the chess board. It was Saturday morning and I took all the kids to Chick-fil-a. The little ones ran around the play area (even the cricked neck boy) and my 11 year old daughter and I played chess. Actually, her older sister and her were teaming up against me. It was a relief when my wife showed up and and took our 12 year old with her to the store. We were able to finish a nice long game in peace. I ended up not loosing any pieces, but she played a well protected game which is why it lasted so long.
That evening, after putting the younger ones down to bed, I fulfilled my promise to play with just my oldest daughter.
There are other interesting coordinate systems. I recently made a "Polar Graphing Friend" for the TI-Nspire. It even includes graph 'paper' that teachers can instruct students to move r to a particular (theta, r) and show them to be sure they got it.
Also there is a page were you trace out the graph in 15 degree increments. This PolarFriend is available as a download on the TI-Nspire Google Group.