Just Bento

Tahini keeps pretty well at room temperature in an airtight container. Otherwise I leave it in the refrigerator. Tahini is not a Japanese ingredient by the way, but it's a lot easier to get a hold of than the thing I substitute it for, nerigoma (rather rough ground up sesame seeds). Tahini can be used in all kinds of Middle Eastern dishes (where it's from).

I think you could probably freeze that. It's has a lot of oil, so it probably wouldn't go rock solid, but the cold would stop the oils from going rancid. I've not frozen it by itself, but I always freeze hummus which contains it. I freeze pesto too; I just put it in iceblock trays, then pop them out when they're hard and put them in a plastic bag. Then I can just defrost useful amounts.

I've used it in tuna fish sandwiches and as a general mayo replacement when I had a lot.

I just found an interesting site that answers food questions and the folks at MaraNatha say you don't need to freeze tahini since it keeps for a year in the fridge after opening. I don't know if you can use that much in a year but it's good to know it lasts a while! Truthfully I think I had the same jar in the fridge for closer to 3 years and didn't notice any bad effects.

In the meantime, to use it up, check online for tahini (or sesame) ice cream recipes, use it in hummus, we like to mix about 3 parts peanut butter to 1 part tahini for our sandwiches and we use it when we make baked seitan.

Here is a link for ways to use it - some of them look bento friendly (especially the tahini tuna which Txaonomist_X suggested too)!
http://mideastfood.about.com/od/dipsandsauces/tp/recipes_with_tahini.htm