Just gonna make a slight stockpile of info for some things you might see in Rapture. Not everything, but SOME stuff.

Items!

Wrench: A basic melee weapon, could be replaced by anything of similar size and dimensions, like a pipe.
Pistol: Everyone gets one of these on every spawn with 18 rounds, so you always have a weapon ready.
Machine Gun: Actually more like a Tommy Gun.
Shotgun: Click-click-BOOM.
Ammo Voucher: Turn in to a U-Invent/Circus of Values machine to get a full stock of one ammo type. You must have found that ammo elsewhere to get it from a Voucher.
Drill: The thing those Big Daddies use. They hurt to be on the receiving end of.
EVE Hypo: Use to restore some EVE.
Med-Kit: Restore some health, but you won't be patching any major wounds with it.
Twisted Nether: A case of Nether energy, with five charges. You may dispense them at will, or throw it like a bomb to detonate the remaining Nether and Twist everything in the blast radius.
Defabricator: Now I wonder what this does...?

Plasmids!

Incinerate: Burn things with a snap of your fingers!
Electrobolt: Nothin' quite loik a fistful ov loitnin', is there?
Vortex: Place a wind trap on the ground for enemies to stumble through!
Sonic Boom: Blasts opponents across the room with a wave of sound!
Insect Swam: A magical hand that SHOOTS BEES!
Shapeshift: Fool your foes... and the Death Note!
Ooze: Suggested by Master4sword, this will turn your body into a gelatinous ooze!
Missile Fist: A magical hand that LAUNCHES ACROSS THE ROOM TO PUNCH PEOPLE!
Telepathy: Speak to others... WITH YOUR MIND!

Tonics!

Shinigami Eyes: See who another character is, and how long they have to live... provided you can read the glyphs that represent numbers.
Elemental Boost: Boost your elemental plasmids for extra damage!
Armor Shell: Reduce incoming damage!
Natural Camouflage: Ever wanted to be invisible? Now you can! As long as you're standing still for at least five seconds, of course.

Enemies!

Splicers: Just your average crazy person... but watch out, because that's no average crazy person!
Big Daddy: Significantly buffed, they should be PLENTY difficult to kill. Usually guards a Little Sister.
Subject Delta: The very first Big Daddy, wielding both a drill and a rivet gun, and able to use plasmids! Works with Tenenbaum and isn't hostile... unless you've been harvesting Little Sisters...
Big Sister: A grown up Little Sister who didn't exactly adjust well, she took up the outfit of a Big Daddy, only more lithe and agile. She can use Plasmids... and she's got an ADAM Needle implanted in her suit. She'll go after you if you've got too much ADAM, make sure you don't get a big head...
Spawn: This should tell you all you need to know.

Bosses!

?????? ???: ... Is that a top hat and a monocle?
????? ??????: Now that's a familiar shape HOLY SHIT IS THAT A SCYTHE ARM?
?????? ??????: This should be a satisfying kill...
?????? ???: Not quite who you think she is... perhaps unkillable?
???????? ?????????: The fallen champion of Headshoots... hungers...
????: The darkness in the eclipse...
???????????: Hahahaha don't even think of fighting this one.
??? ?????? ?????: Something lurks in the ocean's depths outside Rapture...