I keep 7 different types of tape, a simple first aid kit, my laptop, various scripts (I carry ALL of my current scripts, because I'm paranoid about showing up at rehearsal with the wrong script), highlighters, pencils IN BULK that you NEVER want to see again because actors will always steal them, a lighter, tie-lines, safety pins and big kilt pins, and the occasional props - I got a laugh at my last rehearsal for producing two fake cigarettes out of my "bag of wonder" as soon as the director mentioned that we should come up with some rehearsal cigarettes ASAP. I carry shoe glue, crazy glue, glue sticks, 3 notebooks (you know, those new Zwipes notebooks are amazing for keeping a running to do list!), elastic bands, hair elastics, bobby pins, handcuffs (I picked these up on a show somewhere and they have made themselves into a permanent part of my kit.. they arent really neccessary though), a tiny battery operated light and spare batteries, sticky velcro, sticky notes, nail clippers, nail polish remover, and depending on circumstances, walky-talkies, headsets, and batteries for those too. Hairspray is good for removing coffee stains on clothes, or carry a Tide-to-go pen. Quick heat packs, Tylenol, Allergy medicine, wet naps, hand sanitiser, hand lotion, baby wipes.. the list goes on forever. Get tiny travel sized tooth pastes and mouth washes, if you really feel ambitious. It comes down to how much you move your kit around, and how much you love your actors more than the safety of your back.
On my person I keep a multitool, a flashlight (with a blue gel in it), safety pins, bobby pins, elastics, and I keep those little sharpies and some spike tape clipped on my belt.
Theres lots more too, it's show specific. Sometimes I find a disposable camera will do you wonders too, but with phone cams, who needs 'em. Just think "if _blank_ goes wrong, I will need _blank_ to fix it." Try and have a quick fix for everything, but combine as many fixes into one as you can.
Hope I can help.