I've had a bento box for two or three years and haven't used it because it's always been easier to fill a lunch bag, but now I want to cut down on using plastic bags. What are you favorite, more simple things to fill yours with?
If you want non-southeastern Asian foods, which I assume will be easier for you, then the following:
- Cut up fresh fruit. A little thing of peanut butter (or any other nut butter) also goes wonderfully with this. You could also do this with fresh veggies like broccoli, carrots or celery.
- Rice. Here is a handful different furikake (rice seasoning, essentially) you can make at home fairly easy. For the green tea one, just remove the shrimp and fish to veganize it. 
- If you have a Mr. Bento or any other thermal bento, vegan chili’s and this adzuki bean and brown rice porridge are wonderful. With the chili some vegan corn muffins or bread are a nice addition.
- Spicy tofu (seitan works too). Brown some tofu in a pan and then let it marinate in a hot sauce of your choice for however long you feel like. This also works well with duck sauce, teriyaki sauce, peanut sauce and anything that has a strong flavor.
The following are (mostly) Japanese bento items. Most people should be able to use them because they don’t have any hard-to-get “foreign” items.
- Parsnip Kinpira
- Forgotten Vegetable Kinpira
- Two-Colored Spicy Lentil Salad with Cucumber and Pickled Radish
- Red Peppers with Maple Syrup and Chili
- Easiest Ever Carrot-Sesame Salad
- Chana Marsala
There is tons more but there is no way I could fit it all on here. Essentially, anything with a strong flavor will go very well in a bento box. If you have access to a microwave (and your bento is microwave safe) you could put foods that need to be heated up in the bento as well.
Just Bento has a lot of great bento tips. I use it all the time, as well as vegweb and some things I’ve come up with on my own.