Japanese Curry is a favorite food of mine - it's different in taste to curries elsewhere in the world (and I hope you'll get to try it during your travels here). In saying that, curry isn't really the most portable of snacks or meals...but enter the curry pan! Some curry is wrapped in dough to take on a donut shape, then coated with panko breadcrumbs and fried - it's as mouthwateringly good as it sounds. Most convenience stores will have them in their baked goods section - and you can also find them at loads of supermarkets and individually owned bakeries, too.
Take the humble hot dog bun, and fill it with yakisoba noodles - and ta-da...you've got a yakisoba roll. It's perfect for transforming what would otherwise be a cumbersome meal to eat on the go into a transportable snack. You'll often see them topped with pickled ginger or mayonnaise too, for that extra flavor punch! These usually run you around 130-200 yen depending on where you get them - again, just check out your nearest convenience store's baked goods section and you're likely to find them there.
Hopefully this gives you a few options for easy on the go snacks that are wallet and travel friendly - fingers crossed you'll get to enjoy some over the course of your trip here!