Is there an easy way to play music on a iPad through a car's speakers, the way you can do with iPods? (via FM transmitter, cassette adapter, etc)
Yes, all the accessories you mentioned above, as well as any others that work on the iPod, will also work on your iPad.
However, the best way to play music through your car speakers from your ipad without losing sound quality is to use an AUX cable that plugs into your car's stereo. That means your car would need an auxiliary input jack built into your car stereo. I'm not sure what kind of car you have, but many newer cars have the AUX jack. If you don't have the jack, you can get one installed in your car, but I know that can be pretty expensive.