Rituals

1. When summoning demons, have the players actually draw a salt circle around them. If the circle is ever broken, the demon gets in.
2. When a giant, braindead monster comes out, put your cat in the middle of the table. Translate anything she does to the monster's actions. If she knocks over a miniature, they're destroyed. If she goes and sits on the player, they've just had the thing crush them. Poke her with miniatures to simulate the giant's handlers spurring it to action. 50% of the time she might just go to sleep, which is fine.
3. During character creation, get every player to drop a coin over a map (Ideally all at the same time, in a big jangling heap). Wherever it lands is where their character is from. I'm making a game where this determines your race.
4. For a super attack, roll a cricket ball through the battlefield. Any miniature that gets knocked over is crushed.
5. A wizard has shrunk your players into pieces and trapped them in a sadistic board game. Actually bring out a board game, and interpret every event as some kind of encounter. Say, chess: Each piece is some force they can command in any way that makes sense.
6. Arrange a feast in-game, and play it out with using a feast in real life. The players chat in-character as they eat, with you playing all the rabble. Pick one dish: anyone who eats it in real life is poisoned in-game. (Credit to zak)
7. Get some old wooden toy as the Golden Idol. Scratch a clue somewhere on the underside, where it won't be found easily. 
8. Get an old teddy-bear, open it up and stuff a heavy "Gem" inside, then sew it back together. Give it to them as the golden idol. If they notice that it's weirdly heavy and has a bad stitch-job, they can rip it open and find the real prize.
9. If someone's been Silenced, tape over their mouth and force them to do Charades to communicate.
10. Take everybody down to the basement, then turn out the lights and tell them that their torches have blown out and they hear something terrible coming. Flick the lights back on suddenly after there's been some panic: wherever they stand is they are when the monster attacks. (Credit to Gary Gygax - can't seem to find where I read about it, though). 
11. "Dance, dance!" froths the mad king. No, really, dance. Unsatisfactory dancing will result in death.
12.  Get a bunch of lego pieces and blocks that you can stack. Each piece represents one item: The bigger the item, the bigger the piece. You can carry as many items as you can stack. If the pile falls over, you've tried to carry too much, tripped, and scattered everything all over the place.
13. Roll for random encounters every two minutes in real-time.  
14. Get a bunch of matches. These are all the torches you have: You only have light for the time it takes for them to burn.
15. For a sneak attack, throw a dice at your player when they're not expecting it. If they don't get hit, they dodge the attack. 
16. Give them a sealed letter to deliver. If they rip it open, the recipient will know. If they steam it open and re-seal it, no-one will be the wiser.
17. They're caught off-guard in an inn. They only have the items that are around them in real-life to defend themselves.
18. Pick a song for a monster to hum to itself, like this. Put it on loop. Turn it down until it's almost inaudible, then push the volume up higher the closer it gets to the party.
19. To complete a pact, you have to actually cut yourself and smear the blood on a piece of parchment.
20. At the bottom of the worst pit in the world is The Speaking Gun. If you shoot anything with it, throw out that miniature, crush it, burn it. That monster has been completely unmade, now and forever. You can never use it in a campaign again.
21. Man, actually go outside. Write your character sheet on your arm. Trek through a creek, roll dice on streetcorners. The terrain you're going through is the terrain in-game; just point out where the monsters are. 

Inspired by Patrick Stuart.