Make important elements larger than less important display elements (Edwards and Goolkasian 1974), Larger elements are more easily discernible in peripheral vision, which guides subsequent foveal (central vision) fixations. People also typically fixate longer on larger elements in a display (
I learned about this in the article and during one of our class discussions. It's simple. Emphasize the content you know your client deems important and vital for your audience with size and font, but don't get too carried away! Serif is always a great option. Everything else can be in san-serif.