I have two flavors, one for winter and one for summer. I don't have a complete inventory list with me right now, but from memory: socks (1 pair in summer, 2 in winter: socks are perhaps the most difficult clothing item for homeless people to find), a small first aid kit, sun screen, ibuprofen, hand warmers, mylar blanket, toothpaste & toothbrush, a menstrual pad (amazingly useful even if you don't menstruate), a snack that can be eaten with bad or no teeth, a P-51 can opener, a personal letter from me and a sheet listing local resources (including places to get free meals: if someone uses them all, they can eat a solid meal six days out of the week), that sort of thing.
My goal is that the package provides, basically, emergency urban survival items that will last for 2-3 days.
I think the best things to include vary from place to place, so if you are thinking of doing this yourself, my advice is to talk to homeless service providers in your area to get an initial list, then refine over time.
There is a very large homeless population in my area, and about 2/3rds of them are "regulars". I have a pretty fixed territory and the people in my area know who I am enough that I'm no longer really considered an "outsider" anymore. In fact, I often have regulars point me toward people who are in particular need or who are newly homeless.
The care packages are only part of it, though. Another big part is just letting people know that someone out there sees and values them as people, and another big part is just general wellness checks. If someone is having a crisis, others will tell me when they won't tell those they consider outsiders, then I can call in a local agency that will come out to give them a level of assistance I'm not able to. This has happened a dozen or so times now.