Also, the iPad can have more than one language keyboard active. So if you're typing, perhaps en Español a lot, you can add that keyboard and switch back and forth as needed.
Though if you use the accented characters only occasionally, almost all of them are avialable as KevinJS described. Just tap-hold on the keyboard character that most resembles them. Lots of other hidden character as well.
I found this handy when I attempted the Duolingo app.