[junjunforever]

As already mentioned before, syake, yakiniku, tuna mayo, fish eggs (forgot what this was called in Japanese) and umeboshi are the popular ones.

Also, i recommend Omusubi or a mixture of omusubi and onigiri. Omusubi is like adding furikake-like flakes in rice and mixing it together. I personally like this better as it has more flavors to it.

BTW, use the plastic wrap to make and store onigiri/omusubi. I find that to be much easier.