Back in my early gaming days, there was a default gaming kit that I always brought with me in my backpack. They consisted of the following:
The Dice Bag
It was important to bring your own set of dice, and important to keep track of your dice (because of dice thieves in the meta-group at the recreation center I used to frequent, and at conventions).
I used to have favorites among my dice. A small set of three black & red d6s for all my attack and skill rolls. A large set of blue & white d6s for all my damage rolls. A red & blue flecked D20 for any D&D combat rolls. And so on.
I remember having two character binders. One was a 3-ring binder with blank paper and plastic sleeves. Every single character I'd built was kept in these things: one-shot convention characters, long-running characters, just-in-case characters for RPGs that I really like but I've never had a chance to play.
This really depends on the games I plan on playing that night or that convention. Most nights it was HERO, so that meant the relevant rulebooks and sourcebooks. Other times, it would be Call of Cthulhu, or Rifts, or even the Rules Cyclopedia + the Gazetteers during that brief stint that I ran my Karameikos-based campaign.
Earlier on, it was the WEG Star Wars RPG set of books, or the Paranoia boxed set, or the set of little Traveller rulebooks -- or the AD&D triad (PHB, DMG, MM).
These days, I don't bring most of these things anymore. Most games are so rules light, someone else's dice will do, and the character sheets are online, along with the PDFs of the rules.