There are two ways to do this: the first would be to write a web application which you can access through Mobile Safari on the iPod Touch or iPhone. While you say you don't need to store the data on a server, you can use any computer (Windows, Mac, Linux) that can host an Apache server with MySQL and
PHP, and write a web application to add/modify/view data from the
MySQL database. If you also use
PHPMyAdmin, you can directly export the MySQL database as a CSV file which you can import into Excel.
From a software route, this would be entirely free.
The other option is to write the app in an iPod Touch/iPhone native language. You can either download the free
Official SDK from Apple (although if you want to distribute the app to I think more than 5 devices, you will have to pay the $99 to get the app into the App Store)
or you can
jailbreak your iPod Touch (adios warranty) to install unofficial 3rd Party apps. I won't really explain that process here, but a few quick searches from Google and you should find everything you need.