Everything you will ever need in a pool case.

FYI - Links are for pictures and information only.  I do recommend these items but I am not a representative of any of these websites or companies.

Short Version.  Simple necessities to have when going to the pool hall.

1x2 pool case.  Recommendations are for moisture control and solidity of the case.  I like the vacuum sealed cases like It's George, Fellini, Sam Engles, Centennial, or Ron Thomas.  I also like the Dan Whitten style case which master cuemaker Ernie Gutierrez from Ginacue has endorsed in the past.  Essential protection is often overlooked through purchase of cheap cases.  A friend came back from a trip with a cheap case full of cues.  When he took out his cues, he had found they were soaked in water from humidity.  Invest in a great case;  I wouldn't want to have waterlogged cues.
A cue
A backup shaft
Sandman tip shaper - Basically a metal file.
Tip scuffer -  Something that scuffs the tip to hold chalk.  Best I know of are It's George Tip tool, Williard's tip tools, Kamui Gator Grip tip tool, and Brad Scuffer.  I like the Brad scuffer for the portability and key chain hole.  The It's George tip tool has a round circle for shaping and a tapper in the back.
Joint Protectors -  Another overlooked tool.  Several weeks ago, my friend dropped his cue by accident and bent his pin.  This would not happen with joint protectors.  Fancy cue caps are made by Alton Takata.  Simple delrin joint protectors protect well also.

Long Version.  Everything you would need to maximize your pool experience.

Protection/Essentials
3x6 case with long pocket -- It's George 3x6
A cue
A backup shaft
A backup cue
A jump/break cue or break cue and a jump cue -- Recently, Steve Lomax has been the go to guy for jump and breaking needs.
Joint Protectors

Maintenance
Towel
Spare tip, Spare break tip (possibly a leather one thats tournament approved)
Sandman tip shaper

Tip Scuffer

Mushroom grazer

Extras
Cueball - Some pool halls have been using the same balls for 30 years.  The wear and tear changes the way the game can be played.  I like the Super Aramith Pro Cue Ball.
Coin to flip
Crisp new dollar bill - A great shaft burnisher.
Glove - Sometimes the pool hall has 90% humidity.  Sir Joseph gloves have good rep.
Earplugs
Mp3 player
Chalk - Some pool halls have terrible chalk
Magic rack - For practicing, inability to rack, or figuring out breaking secrets.
2 Pens, 1 notepad - Never know when you need to write something important down.
Ibuprofen
Candy/Protein Bar
Justa Bridge - A custom bridge that has cloth covering so it doesn't scrape your cue.
Cue extension - Sometimes it's just easier to to reach a shot than to use a bridge.  Popular cue extensions include Longoni, Tony Ryan, and Mezz.