There are sleeves that you can pull on w/your t-shirt that will give you sun protection. Athletes often wear them. I don't have any reccomendations for a particular brand, but I'm sure you can find something on Google.

I wear lightweight 100% cotton shirts over tank tops. I take them off when I get too warm and either tie them around my waist or put them in my bag. The hoodylicious (yeah, ignore the name) from Athleta is very lightweight t-shirt material.
http://athleta.gap.com/browse/product.do?cid=85909&vid=1&pid=553250002

How about getting a sun protection shirt with removable sleeves? Google should be able to point you to specific sites and brands.

We will be doing a big trip of national parks on the west coast this summer (July). While I can dress fine for outdoors for cold weather, I am looking for something to cover up my arms in the sun. I hate the sunscreen and stickyness of it.

I bought a couple of pairs of pants from REI that serve me well to keep my legs covered. Can you suggest a light weight jacket that I can wear in hot sun? I don't want long sleeved shirts because I want to be able to take the long sleeves off when I am in shade. I don't mind the sweat caused by two layers. My guess is light colors are better than dark ones is that right?

Thank you.