Change number of desktops: right-click on an empty area on the desktop, click on Desktops and then Add or Remove Desktop.

Change keyboard layout (multi-language)

Change layout now (in terminal):

sudo setxkbmap -option grp:shift_alt_toggle gb,gr

Set kb layout permanently (taken from this forum):

For a (permanent) uk/greek keyboard configuration, using alt+shift for language switching:

sudo mousepad /usr/lib/X11/xorg.conf.d/05-evdev.conf
(replace mousepad with your editor of choise)
 and modify

Section "InputClass"
Identifier "evdev keyboard catchall"
MatchIsKeyboard "on"
MatchDevicePath "/dev/input/event*"
Driver "evdev"
EndSection

to

Section "InputClass"
Identifier "evdev keyboard catchall"
MatchIsKeyboard "on"
MatchDevicePath "/dev/input/event*"
Driver "evdev"
 Option "XkbLayout" "gb,gr"
Option "XKbOptions" "grp:alt_shift_toggle"
EndSection

Save and exit.

Take a screenshot - use Scrot (preinstalled in Lubuntu)

Command in terminal (5 seconds delay):

scrot -d 5

Change default web browser (taken from this forum)

sudo update-alternatives --config x-www-browser