The stuff everyone already loves
Bluetooth is the obvious one. Plug your phone in once, and from then on it just works, calls, music, the lot. Reversing cameras are the same story, switch the car into reverse and there it is, no thinking required. Built-in navigation and heated seats or windscreens fall into the same bracket. None of these need explaining twice. They solve an obvious problem the second you use them, so people use them.