It all depends on what deity you are leaving offerings for.
This is probably a personal decision, so in the end it's up to you, but here's some advice that I've heard over the years:
Try giving that deity an offering that relates to their attributes. Or, more simply, mix together some herbs and leave them out. Leave them a note with your thanks written on them. There are many things you can do, but I'd recommend using anything from nature that can be sent back to the Earth. Blessings!
I am trying to give to give an offering to a spirit that I work with a lot. I want to do it as a way to say "thank you for helping me". I'm not worshipping this spirit and they would feel uncomfortable if I even thought about it and to be truthful the thought of worshipping them makes me uncomfortable too. I rather honor a spirit/deity and show them respect than to worship them.
Go with incense. Avoid those with cleansing properties. Ask the spirit if there's a certain scent they like. Light it, say it's to thank them, and place it on your altar or shelf or wherever. Just keep it in the same place every time. People light incense offerings for their ancestors, deities, faeries, spirits, and whatever else they work with. Just specify who it's for.