Adventure Mode is Gamemode 2.
/give 137 makes a command block, one that can give you items, with the right cheat inputs.
An iron golem will get hurt when built against a wall.
Sometimes, a spider and its jockey will separate.
"/gamerule doMobSpawning false" that turns off natural Mobs spawning, excluding spawners and seiges. (Perfect for PVP maps, like "The walls")