Here's some of my achievement ideas:
Killing Sprees:
The Beginning - Earn your first kill with any weapon.
Bloodthirsty - Kill 3 people in one game, with any weapon.
Death Dealer - Kill 5 people in one game, with any weapon.
The Slayer - Kill 7 people in one game, with any weapon.
Grim Reaper - Kill 9 people in one game, with any weapon.
Mobile Massacre - Kill 13 people in one game, with any weapon.
Sharpshooter - Kill 2 people using a bow, in a row.
Straight Shooting - Kill 4 people using a bow, in a row.
Sniper's Dream - Kill 7 people using a bow, in a row.
Fire! - Kill 2 people using flint n' steel, in a row.
Burnt to a Crisp - Kill 4 people using flint n' steel, in a row.
The Pyro - Kill 7 people using flint n' steel, in a row.
Miscellaneous Kills:
Cliffhanger - Kill somebody by having them die by fall damage.
Boxer - Punch somebody to death.
Picking To Death - Kill somebody with an iron pickaxe.
Digging Their Grave - Kill somebody with an iron shovel.
The Warrior - Kill somebody with an iron axe.
What A Farmer Does in His Free Time - Kill somebody with an iron hoe.
The Magic in Me - Kill somebody by hitting them with a Golden Apple.
Catching Fish - Kill somebody by hitting them with a Fishing Pole.
Tickling A Tribute - Kill somebody by hitting them with a feather.
Execution - Kill somebody by having them die by lava.
Need... Air..! - Kill somebody by having them die by drowning.
Death in the Match - Kill somebody during deathmatch.
Quick and Clean - Kill somebody in the first 10 seconds of the game.
Ownage - Kill 1 staff member.
King of the Hill - Kill the number 1 player on the leaderboards.
Finish Him! - Hit a player with a sword, then finish them off with one arrow shot.
Killjoy - Kill somebody who has earned 7 kills in one game.
Killjoy 2.0 - Kill somebody who has earned 11 kills in one game.
Caved in - Kill somebody by having them die by suffocation.
Driving By - Kill somebody while in a boat.
Food:
Real Men Eat Things Raw - Eat any raw meat.
Sickness - Get sick off of eating raw chicken.
Chef - Cook 2 pieces of meat in one game.
Five Star Chef - Cook 10 pieces of meat in one game.
Professional Chef - Cook 30 pieces of meat in one game.
Power! - Eat 1 Golden Apple.
More Power! - Eat 5 Golden Apples in one game.
Massive Amount of Pooowwwweeerrr! - Eat 10 Golden Apples in one game.
The Love for Wheat - Craft 2 pieces of bread in one game.
The Love for Apples - Craft 2 Golden Apples in one game.
Steak Man - Eat 5 pieces of steak in one game.
Pork Man - Eat 5 pieces of cooked porkchops in one game.
Chicken Man - Eat 5 pieces of cooked chicken in one game.
Sweetheart - Eat an entire cake.
You Can Haz Cookie - Eat a cookie.
Poisonous - Eat a Spider Eye.
Points:
Bounty Man - Earn 300 points.
How to Be Rich - Earn 1,000 points.
The Big 3,000 - Earn 3,000 points.
The Halfway Point - Earn 5,000 points.
Over 9,000! - Earn 9,000 points.
Earn ALL THE POINTS! - Earn 10,000 points.
Takedown - Bounty 300 points on a player, and have them killed.
Determined for Death - Bounty 1,000 points on a player, and have them killed.
Desperate for Death - Bounty 3,000 points on a player, and have them killed.
Big Spender - Bounty 5,000 points on a player, and have them killed.
The Look on Their Face - Bounty 10,000 points on a player, and have them killed.
Loot - Kill a player and earn 50 points.
Treasure - Kill a player and earn 100 points.
Lottery - Kill a player and earn 250 points.
Jackpot - Kill a player and earn 500 points.
Victories:
Last Man Standing - Win your first game.
Used to Winning - Win 10 games.
Trouble With a Tribute - Win 50 games.
Unstoppable - Win 100 games.
Star Player - Win 300 games.
You Shall Not Win! - Win 500 games.
Glory of the Game - Win 1,000 games.
I put a lot more than I thought, if any of you actually read all of that, here's an achievement for you.
A Good Reader - Read all of this post.