I usually carry parchment (to write quick spells) and string (string magic) in my wallet and i have a mini spell kit of a few herbs and crystals in both my car and my locker at work (and i dont even necessarily always use physical items in my spell work.)
i always think preparation befor a spell is good, but sometimes you just need it there and then

p.s. love the sage thing, i got ask if i had been smoking marajuana once when i forgot to shower after doing a spell before going to work
The parchment and string is a good idea, I usualy write on sage or thin cedar wood chips, but my S.O. has a small wallet so he can't carry the wood chips.
I used to carry an mini alter that included a copper penny, a twig, a small exacto blade,9 metal soda caps, and seven color coded birthay candles,a quartz crystal,a small cone of inscense, a mini alter cloth, and it all went in a little wooden box about the size of soap box.
I could set up for an entire ceremony anywhere! Of course this was many years ago before I learned how to do ceremony wiyhout the props.

My friends used to joke with me and say I should make a wiccan barbie and include a little alter like that. I just shrugged them off because I had several meaningful ceremonied with my alter to go.
I also have some ghost blend(salt, pepper, sage,) and a smoky quartz that fit into one of those little emergency pill containers that can go on a keychain. I mady my S.O. one too, and he's actually used it when he felt some icky energy hovering arround his truck after work. The next night the truck next to him got broken into, so after that I make him carry one with him everywhere.