Hair, definitely. I'll also add -- lipstick. Some color on my lips goes a long way, especially as I'm aging.... and fading. It can be as simple as a tinted lipgloss. Just to add a bit of color.
Also just feeling comfortable in my clothes, nothing pinching or binding or making me feel fat. Clothes that fit, regardless of what my size is. This includes my shoes -- well fitting shoes that don't hurt. I never feel beautiful if my feet are aching and full of blisters.
Having my nails painted. It puts a smile on my face. Makes me feel pampered and cared for. I never thought I'd say that, as I went for decades without ever painting my nails, and didn't start getting regular mani-pedi's until about 3 years ago.
Wearing my contacts instead of my glasses. I feel ugly in my glasses, and like there is a window between me and the world. (Sunglasses not included -- I love sunglasses).
Taking a long walk and having pink in my cheeks.