For insulating stuff: dry socks, waterproof boots and gloves, blankets, sleeping bags, jackets, CLEAN cardboard layered several inches thick and sealed in plastic (wrap or trash bag) for use as a make shift sleeping pad.
For shelter: tents and tarps are always in demand. Just cutting the wind will help a lot and a tent can provide minimal insulation.
For active heating: Sternos, candles (once the wax is melted mix in hand sanitizer for a warmer heat), those hand warmer things, propane camping stoves (DANGEROUS. They are very effective, but 100% will be used in tents or under tarps which can very easily cause a fire or ventilation issues), you can also buy some 1lbs propane tanks and fill hose to siphon gas from a larger 20lbs tank (this might be illegal where you are).
You might want to check if any moving companies have some old blankets they are willing to donate (used to protect furniture, but if they toss them when they wear out can be useful for warmth or bedding). Also call up local theaters, zoos, aquariums, etc and ask if they would be willing to donate unclaimed lost and found clothing. This can be a good source of jackets. Also worth seeing what’s in your thrift stores, especially if you have a smaller local one instead of Goodwill.
I recommend distroing cinder blocks along with the Sternos or candles. You can place those inside the block (so the flame hits the inside of the top) which helps contain the flame and resists getting knocked over. Another option is to hot glue some empty cans to a large flat piece of plywood. This cannot tip over even when kicked so it will provide a simple place for the heat producer to go. Just keep in mind not all unhoused people will want those safety devices as they are heavy and cumbersome, so don’t get disheartened if they are turned down, but it doesn’t hurt to have a few available and be prepared to demonstrate.
If you do choose to provide candles, Sternos, or ovens, be sure to also provide matches or lighters.
Oatmeal also is pretty easy to make and can serve several people. I use a stock pot and can cook about half a large container in one go, just dump it into some sealable bowls or whatever and serve it out of your trunk.