Here's something for you that takes no time, needs no equipment, is challenging and can be repeated daily.
Set the timer for 3min and do as many push-ups as you can. Try and be strict with your form. Repeat every day and watch yourself improve.
Everybody has 3min and everyone can do at least 1 push up.
Great little challenge that is accessible to everyone.

*Can substitute push up for another exercise e.g burpees.