If you own a laptop computer, then there's a good chance that it's equipped with a CD player. Why not utilize your computer as a portable music center?
One of the advantages to using your computer is that you have three ways you can listen to your CDs. You can choose to use a set of headphones connected to the headphone jack for a more personal music experience. This method probably works better than the second option, which is to listen to your music through portable speakers, also connected through the headphone jack. One drawback to this method is that in addition to having to take your laptop computer with you wherever you travel, you'll also have to carry a small, yet bulky set of speakers. To circumvent this problem, you could leave the speakers in your car and hook them up each time you wish to use your computer as a portable CD player.
The third method, which may be the least desirable option, is to use the laptop's built-in speakers to listen to a CD. Your experience using this technique may vary, as it greatly depends on how audible your laptop's speakers are combined with the amount of ambient noise you can hear while your car is on the road.
If you choose to use headphones, make sure to check that it's a legal option in your area first.