vanilla, coffee, tea, some spices,
tobacco are the main ones that can be extracted. Some people really like them and it is much simpler than you would think. The only part with a degree of difficulty is straining. people are using filter paper and pump setup to force the mixture through the paper and remove as much as possible from the alcohol/PG. Syringe with cotton also works, rough filtering through an unbleached coffee filter, you get the idea. Known coil gunkers, but you can also mix and vape something no one else