1. The Mob Arena: Get some dispensers and a couple of redstone repeaters and some redstone. Set up a simple redstone clock and put some dispensers completely full of mob eggs. Then you connect the clock to the dispensers and start up the clock and their base will soon be filled up with whatever mob you chose.
2.The Explosion: Give them a pressure plate for their door and put TNT under the block that the pressure plate is on.
3. The TNT Long Shot: Make a long redstone trail to their base and put TNT around the back of their house. Then, once they are coming over to do something to your house you can activate the wire to blow up their house.
4. The Invitation: Ask them to come over to your house and then say you have to get some wood for the fire or some excuse. Then once you leave, go to their house and fill it with lava.Or sheep!
The New Block 
Instead of using a clock mechanism I use a rapid pulser one end linked to the dispenser and the other end a switch . I have also done no. 2 but not 3 or 4 Will try them though
Instead of using a clock mechanism I use a rapid pulser one end linked to the dispenser and the other end a switch . I have also done no. 2 but not 3 or 4 Will try them though