I find really useful to switch keyboard layout while holding a key. That way I can easily type an English while writing in Greek. Left Windows key is good option for me as I don’t really need it on my Arch Linux powered laptop.
Can be easily setup in
Section "InputClass" Identifier "keyboard-layout" Driver "evdev" MatchIsKeyboard "yes" Option "XkbLayout" "us,gr" Option "XkbOptions" "grp:alt_caps_toggle,grp:lwin_switch" EndSection
A full list is available in