A friend dropped his Dreamcast off at my workshop (cluttered desk covered in dirty PS2 controllers). He needed a new CMOS battery in his Dreamcast because it wasn't storing the date properly anymore, even after being charged with regular play. I replaced the battery with a new one and also threw in a little artistic bonus.
The Dreamcast is unique because it has an ML2020 battery to store its date and time. It's a small coin cell, about the size of a regular watch battery and around the same voltage as those (3v). It's rechargable though. As the system is on and playing games, the battery trickle charges. After a while though the battery will not charge as well, and eventually, the Dreamcast will no longer be able to store its date, time, and the player's birthday.
Replacing the battery is a simple process, and all it takes is a new ML2020 ($5). Because it was open anyway, I decided to replace the LED with a soft-white diffused one. I think that it matches the rest of the system quite well with its sleek, black exterior. Looks very NeXT or even IBM late-80's if you ask me ;)