Sudoku?
Learn a new language?
Learn something else you always thought you'd like to?
Build a model ship/car/rocket/airplane?
Design your own rocket launcher?
Plan my next vacation for me?
Draw or paint a picture?
Knit a scarf?
Think about something that's been problematic, to which you've always thought "I wish they would....." and invent a solution for it?
Write that book you always knew should be written?
Record yourself singing that song you know you sing so well?
Do ANY of those things interest you?