Here's a list of all the content in PR3R. This isn't everything of course, just the main types of content.

Hats
Police Hat: Adds an additional 2 shots to laser guns.
Ushanka: Immunity to ice. Full stats while standing on ice.
Toque: Damaging players causes them to freeze for a moment.
Fez: Rotation blocks are 10x as fast.
Witch Hat: Attract hats on the ground. Pedro is pushed away.
Halo: Start the match with Angel Wings. Getting damaged causes you to lose them.

Items
Bow: Shoots an arrow that arcs based on the amount of time that you charge it.
Napalm: Become engulfed in flames for a few seconds and damage anyone who you touch during it.
Super Teleport: Like PR2's teleport, allows you to teleport past 3 blocks instead of 2.
Snowball: Three throwable snowballs. Does no damage, fires quickly, freezes whoever it hits like Toque.
Freeze Ray: A long beam that captures whoever it hits in a block of ice. The player in the block of ice slides around and can't move for a few seconds. Getting damaged breaks the ice early.
Heart: Heal 1 HP in deathmatches. Allows you to heal above the max HP amount.
Lightning Cloud: A cloud of lightning appears above your head. Touching players passes the cloud to them. Whoever ends up with the cloud after a few seconds gets zapped by lightning.

Game Modes
King of the Hat: The level creator chooses a hat and chooses where to place it in the level. The players have to wear that hat for a certain amount of time to win.
Damage Dash: The players gain points by dealing damage. When the time runs out, whoever dealt the most damage wins.

Blocks
Heart Block: Heals HP in deathmatches.
Reflector Block: Reflects projectiles.
Weak Block: Slowly crumbles as you stand on them.
Glass Block: Acts like a weak block on top, and a reflector on the sides.
Code Blocks: Allows you to do various cool things with its own custom coding language.
Checkpoint Block: Overrides safe blocks. Safety nets teleport you to these instead if they've been touched.
Buttons and Reactors: Touch a button to shatter the corresponding reactor.
Dispenser Block: Dispenses coins in coin fiends.