I have frozen "silver rolls" - (steamed bread rolls), Naan, and mixed frozen veggies. since most of our bento are made from dinner leftovers, and the chest freezer is mostly full of frozen chicken and food for the critters, there's not a lot of space for bento staples for us!
However, since we eat rice for dinner at least a couple nights per week, there's almost always leftover rice to do something with. I've also been known to cook up ramen noodles in the morning to drop in a bento. There is almost always cheese in the fridge, along with fresh fruit.
We also keep nuts and dried fruit around to fill in spaces in bento and tummies.
I have a block of paneer that I need to do soemthing with, so I may cut it up and fry it, and then re-freeze it to drop in Indian sauce leftovers...mmm...