An example creating an activation link which allows you to set your own password.

diy-activation-link.php

<?php
/*
 * Bootstrap this outside of normal wordpress plugin
 * won't load theme support functionality
 */
define( 'WP_USE_THEMES', false );
require( './wp-load.php' );
/*
 * Need to know the ID for your user.
 * #1 is usually an admin. Don't put this on a live site.
 */
$id = 1;
/*
 * Creates a WP_User object. It needs the user id.
 */
$user = new WP_User($id);
/*
 * These are the args we'll need to create a password reset url.
 */
$options = array(
    //reset password action
    'action'=>'rp',
    //generate a new key just for this user
    'key'=>get_password_reset_key($user),
    //The user "login" name
    'login'=>$user->user_login
);
//Our completed password "reset" url. http_build_query turns our array into a url-encoded name/value pairs.
$url = get_site_url() . '/wp-login.php?' . http_build_query($options);

echo "Password reset link for user $id is <a href='$url'>$url</a>";