How to hide the User Interface
From Second Life Wiki
To make the whole User Interface (UI) disappear, you can either use a shortcut or through the advanced menu.
Press- - - to make the Advanced menu appear.
Then go to 'Rendering', unclick 'Show UI'. That will make all the UI disappear. If you want to get the UI back use the shortcut:- - .
- - can also make the UI disappear.