There is so many fun flavor, like goma (sesame), sakura, matcha, hojicha, ramune. I took a picture of a small stall where they had like 40 kind of soft cream. The secret is that machine is not freezing a liquid, but the ice cream is already frozen an packed in single serve container, that is squeezed to get the ice cream out. Kind of like a coffe machine using pods. So it's easy to have many flavors in a small ice cream stand.
I also remember that one of my friend told me about strang ice cream flavor at Nanja Town in Ikebukuro, like octopus and crab ice cream. Might not be soft cream, but 100% unique flavors.
If it's just for the wow factor (like the gold leaf ice cream), then there is the super tall soft cream you can find in America Mura, Osaka. Just plain choco or vanilla ice, but you get something like a 30 cm tall tower of soft cream.
Edit : I just remembered, sweet potato flavored in Kawagoe, there is the murasaki-imo (purple) and satsuma-imo (orange). That's something fun to taste.