Money is the traditional gift at a Japanese wedding. The amount depends on your relationship with the couple (ask another guest if you are not sure). Give it in an envelope called a shugi-bukuro with your name on the front. Use new, non-creased bills if possible.
Traditionally, you are to bring cash to a wedding as a gift. The cash gift is called an Oshugi and is put in an envelope called a shugi-bukuro. Sometimes the couple will tell you how much they want in the invitation itself.