The wonderful thing about mochi is that you can put anything inside. If you need some inspiration you can go with chocolate ganache, ice cream, fruits, peanut butter, crushed cookies, jam, sweet bean/seed pastes, and even savory things. The only caveat is that you probably want whatever you are making to look good, so I would stick to foods that you know will keep the shape you want.
Sesame Peanut Butter – I didn’t have sesame paste in my house so I crushed sesame by hand with a mortar pestle and mixed it with peanut butter. I used about equal parts of both because I didn’t want a very sweet mochi (with too much peanut butter), and when you use more sesame it keeps it’s shape better.
Chocolate Ganache AKA Chocolate Truffles - It’s pretty straightforward to make a simple chocolate truffle; it is so simple that you’ll possibly not buy one ever again. You have two basic ingredients: chocolate and milk.