1- What is the awrah for men which is obligated to cover?
2- Is it permissable for men to wear t shirt and shorts that cover the navel down to the knees, but do not cover the knees itself? I have heard that it is recommended in prayer to cover from navel to knees, but not an obligation. Is it the same for regular outdoor use as well?

1 – Awrah is private parts
2 – Yes