It'd be cool if when not using a tool, clicking would pick up a physics brick in front of you and let you move it around until you use a weapon or click again. It could be handled mostly client-side: The server just sends "X tried to pick up a physics brick" to everyone, each client copy does a ray-test for a physics brick in front of that player's view and if so makes it hover in front of them until dropped. If the physics simulations are different from one client to the other by a large amount then it just wouldn't pick up anything.