Option+e gives an acute accent: é, ó, ú, á.
Option+u gives an umlaut: ü, ï
Option+n gives a diacritical tilde: ñ
Option+i gives a circumflex: î, â
I learned most of this by looking at the OS X's virtual keyboard. When you press Option, it has all the things you can do with the keyboard. It's pretty fun actually, and useful! It really helps with math symbols too. For example, Option+w gives ∑. There's a lot of options. That's partly the reason why I disable Press and hold "feature". It really makes it harder to type fffffffuuuuuuuuuuuu