/**
 * Adds a "New Content" activity dropdown filter.
 *
 * This filters the activity loop by new blog posts, blog comments and forum
 * topics.
 */
function my_bp_add_custom_activity_dropdown_filter() {
    $types = 'new_blog_post,new_blog_comment,bbp_topic_create,new_forum_topic';

?>

    <option value="<?php esc_attr_e( $types ); ?>">New Content</option>

<?php
}
add_action( 'bp_activity_filter_options', 'my_bp_add_custom_activity_dropdown_filter' );