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!