Bokashi fluid is the last one of the easy natural fertilizers I've found. There are more plants and stuff to use, but they are somewhat harder to handle or are more specialised, so I've decided to leave them out for now.
Bokashi is, as you probably know, a way to pickle kitchen left overs. The friendly microbes drissled over the scraps splits the food into its basic compounds and makes it easy to break down in the soil or in a compost. During that process fluid is released (it's contained in the food from the beginning) and must be drained off. My own bokashi buckets have a tap in the bottom for that purpose, which makes things easier.
The fluid is a good fertilizer. Dilute it one part to hundred and you have a humdinger for your potted plants. My tomato plants - which are nearing their one year aniversary so I'm planning to take them down - suddenly grew lust for life and set fruit again when I used it. But it's uneconomical to use outdoors - you get less than a deciliter (less than 1/3 cup) per week and you have to use the fluid within twentyfour hours.
Bokashifluid is a fertilizer I recommend for indoor gardeners for several reasons; it's good for 'hungry' plants, is easy to get if you have invested in bokashi buckets, and it smells very little. In addition to that the pickling feature of the bokashi means you can give meat and protein scraps a better purpose than to be sent to the landfill. If you are growing plants indoor you should definately buy a bokashi.