The "US international" keyboard layout makes many special characters available with some key combinations. A few commonly used ones:

Character Key combination
° [AltGr] + [Shift] + [;]
å [AltGr] + [w]
ç [AltGr] + [,]
č [AltGr] + [Shift] + [.], [c]

The keyboard layout can be shown in a window using gkbd-keyboard-display -l <layout>, e.g. gkbd-keyboard-display -l us-intl.

Sources