I think this question depends greatly upon two things; your experience level being the #1 biggie, followed by the type of "theme" or "genre" you work from.
When I was working in the Playboy clubs and similar environs as a Close-up magician I kept things very simple; a few gaffed coins as well as 5-6 real coins, a deck of cards, my sponge balls and depending on crowd size, etc. I may have a packet trick or two (Color Monte being a proven standard alongside The Gypsy Curse) as well as my Cups & Balls. That's pretty much it other than maybe a special odd or end like an ITR, TT, etc.
As a Mentalist/Psychic Entertainer I rarely carry more than a John Rigg's Butterfly pad (actually, mine is heavily modified so that I can do more with the one unit) a hand full of billets, some golf-pencils. Recently I've made it a habit to always have my
MAO TOU Pendulum in that it's the single greatest tool for serious "old school" mentalists to come along in decades. On top of this I may have a deck or partial deck of tarot cards with me depending on the situation.
The big thing when it comes to Mentalism, is that you don't need a lot of stuff. As "outs" I will always have a swami and an SUC Case set for Pocket Writing but it is exceptionally rare to find me with any kind of special wallet and even then it's most likely a cheap Himber styled hip wallet and maybe Docc Hilford's Serpent Wallet. Most of what I do is "legit" (gimmick free) in that I rely a great deal on Ideomotor cues (not just Muscle Reading) and of course, my knowledge in how to work with most Oracles (Palmistry, Numerology, etc) coupled with my (for lack of a better term) "cold reading" skills.
SOMETHING TO THINK ABOUT. . . if you're a fan of Mentalism try backing off on how much material you do and what you do. Focus more on what the public wants to experience vs. what you want to show off with. If you make your material personable to the group members you are performing for, they won't only remember you, they will tell others about you. They won't call you a "Magician" but credit you for being a Mystic, at minimum but most certainly a "Psychic". Like it or not, that's where the money is at.
and on special occasions I'll pack a deck of Tarot, my Pendulum