In your plugin class (e.g. MyPlugin), add public static function, returning some content:
public static function my_plugin_output() {
// Build your content
$output="<div>";
$output .= 'This is a text to display.';
$output .= '</div>';
return $output;
}
Once your plugin is activated, add this line to any template, you want the plugin content to appear:
<?php if (class_exists('MyPlugin')) echo MyPlugin::my_plugin_output(); ?>