1 player. Experiment with your instrument’s technique and make a list (mental or written down) of those things that are (very) easy to do technically, such as creating a dense flurry of notes by simply wiggling fingers on keys in a certain way or in a certain order while blowing. Looks/sounds hard but is easy to execute. Then create a piece around them.
Variation: Make a piece with the form A B A, with A being a fast section that uses all the techniques that are easy on your instrument and B a slow section that uses techniques that are difficult on your instrument.
Idea: Get together with a friend on another instrument who also played this game and make a piece together. Try different combinations: Both easy. Both difficult. You easy, Them difficult. Them easy, you difficult. Record it all!