Always carry a pen, a USB memory stick, a pocket umbrella, a bottle opener, a condom (some of these examples may not apply to you) etc., as you may find yourself needing one without knowing beforehand.

I do not regularly carry any of the items on this list and have not felt the need to begin carrying them.

My default set of things to carry with me consists of three items: a wallet, a phone, and a set of keys.