There are many kinds of Japanese sweet buns, called kashi-pan in Japanese.
The three introduced here are the most famous and widely available at bakeries, supermarkets, and also convenience stores in Japan. They are reasonable (around 100 yen) and tasty. When you get a bit hungry in traveling, these buns will help!
>> An-pan

An-pan is a Japanese bun first sold in 1874. It's called an-pan because there is an, which means sweet bean jam, inside of the bun. There are different kinds of an-pan with different types of bean jam.
Also, there is a popular animated cartoon, "An-pan-man," where a hero modeled after an-pan saves others against "Baikin(germs)-man." An-pan is such a popular food in Japan.
>> Cream-pan

There is custard inside of cream-pan. It's popular especially among children. It's said that this bun was developed by a baker who was impressed with cream puffs. The kind of custard and the amount of it is different.
>> Curry-pan

This is a non-sweet bun. It contains curry inside and is fried or baked. It also looks like a piroshki, a Russian bun.
The curry inside of the bun is usually mild, and the vegetables are cut to pieces. Curry-pan tastes good but a bit oily.