Arm and leg warmers for sure. I always was bothered by the "chilly chest" syndrome while riding. I tried a windbreaker jacket, but it was bulky and difficult to store and carry once it got warm and was removed.
Then a woman I rode with tipped me off to Tyvek. Yup, the stuff they wrap houses with. There's always sheets of it being discarded near any home construction site (and there are too many of them around here lately.) Just grab a piece, cut it to size to fit between a light t-shirt and your jersey. It works better than any dedicated garment I've tried before. When it starts getting warmer, just remove it. It will fold into a small package you can fit in your jersey pocket, or just find a dumpster or trash receptacle for it and cut a new one next ride.