These are the items I frequently check for when traveling. Some, like ID, are critical to travel. Others are conveniences that I’ve overlooked so often that I feel dumb when I leave them behind. 

ID, passports, wallet 
Electronics— cell, laptop, e-book
Chargers.
Water bottle
Weather appropriate jacket
Weather appropriate hat

I’ll do an explicit check when I leave my home or hotel— I must see each item. Other times, such as when I am getting out of a taxi, it’s enough to mentally go over the list.

When I am traveling with someone I’ll occasionally double check on their items. So many people leave water bottles behind. 

I should probably add sunglasses to the list, except I’ve lost my sunglasses so often that I don’t bother with them anymore 

I have a collection of caps and hats that only exists because I’ve repeatedly forgot to bring one along and so bought one at the destination. 

Once I left my e-book on the jet. Two months later, the airline reached out to me and returned it. That was luck.

Chargers... I have left so many chargers behind. The other way around, I’ve discovered many stray chargers in hotels and guest homes. Every now and then I’ll stay overnight at my brother’s before I fly out of Ottawa. The cell charger in his spare bedroom is one I forgot years ago.