Grey Boxes item lets you spawn 3 boxes individually behind you for other players to trip on. Try to place them in annoying places so other racers don't have a good chance of dodging them, or dropping them onto others from above.
The Golden Hook is pretty much the Blue Shell of Speedrunners. It's a powered up grappling hook that will attach to a player ahead of you, and pull them behind you while propelling you forward. If you're quick enough, Slide before it grabs you to dodge it. For best effect, use it when both you and your target are in the air.
The Shockwave is a pretty effective Area of Effect weapon. It takes a second to fire, but once it does, anything inside its large range will be affected. Any props will be knocked off screen, any weapons (like rockets) will be destroyed, and any players will be knocked back depending on where they are in it.
The Magma Rock will launch a massive burning boulder in the direction you're facing, knocking over anyone who's unable to react in time. If you're in first, turn around quickly and launch it at everyone following you to separate yourself further.
The Remote Explosive does exactly what it sounds like. First place the trap, and then press the button again to detonate it when it's worst for your enemies. Like the crates, this can be used to completely shut someone down when used from above.
Lastly, the Invica-drill. Use this, and for a brief period of time you'll become a very fast drill that will speed right through anything. Destroys props, blocks weapons, and knocks players out of the way. Try using it while boosting!