Add a string after each item in WordPress sub menu

You can do this in three ways:

1) Use before and after arguments in wp_nav_menu

2) Write a custom walker

3) Use CSS :before or :after pseudo classes to add the character. The character must be in escaped unicode, e.g.:

li:after {
 content: '\00BB';
 padding-right: 5px;
}

There is more in a similar StackExchange answer