Is there a car-friendly way to play music on the iPhone without a car kit?
You could get something like this remote, which has passthrough for any standard 3.5mm audio, and provides volume controls and a clicker remote (just like on Apple's headphones). The clicker can to play, pause, and skip (double click for forward, triple click for backward).

There are other similar products out there, I found a few by searching for "iPhone inline remote".