A trick I learned in my photography class is to use your hand as a gray card. Meter to your hand and then add 1 click more light, either shutter speed ofr f-stop. This is handy outside and gets you generally to the right exposure. Sometimes I have to fine tune to get what I want. The only problem is, people will thing you are strange trying to photograph your hand

Also if you are outside, green grass can work as a gray card if needed.
Susan