This is probably a cheeky request. Any possibility of adding a gun so we can shoot stuff at the boxs to see them move around and fall. Maybe they should just fire paint pellets.

Btw, to keep the Mouse focused within an SDL Window you should use
SDL_WM_GrabInput( SDL_GRAB_ON );

or

SDL_WM_GrabInput( SDL_GRAB_OFF );
To turn it off.

you can also use
mode := SDL_WM_GrabInput( SDL_GRAB_QUERY );
to find out what is the current GrabMode.

I would also suggest using SDL_ShowCursor( 0 ) to hide the cursor, to avoid the distraction caused of it appearing and disappearing.