Simple:
* He was so absentminded he forgot to bring his lunch to work.
* The professor, known for his absentminded nature, often walked into the wrong classroom.
More descriptive:
* Her eyes glazed over as she stared out the window, lost in thought, a classic example of absentminded behavior.
* He was so absentminded that he often left the stove on, unaware of the potential danger.
* She was so absentminded that she often wore mismatched shoes without realizing it.
Figurative:
* The dog's absentminded scratching at the door reminded him of his forgotten errands.
* The city's absentminded disregard for its parks was evident in the overgrown weeds and crumbling benches.
Formal:
* The absentminded professor, lost in his own world, failed to notice the student's raised hand.
* Her absentmindedness was a constant source of amusement to her friends.
Remember that "absentminded" describes someone who is forgetful or preoccupied and not fully paying attention to their surroundings. It can be used to describe both a person and their behavior.