I got a 4GB iPod mini a little over a year ago, and I love it. The mini has been replaced by the nano. (My husband has a nano. I have iPod envy.
) If you don't have a whole lot of music, and small and lightweight is important to you, the nano might be a good choice. Otherwise, go for a full iPod.
For listening in my car, I use a cassette adapter. The sound quality is pretty good. This setup would be perfect if you usually drive rental cars, provided that the rental cars have cassette players.
Edit: Be warned that if you order an iPod through apple.com, they will ship FedEx, signature required at time of delivery. I learned this the hard way after FedEx returned my husband's iPod to Apple because no one was home to sign for it. Apple could have warned me when I placed the order, but noooo
I suggest you have it shipped to your office if you order online.