When we went last Oct, I decorated the handle with an orange bandana from AC Moore & then hung a personalized Halloween Themed stroller tag from it. For costumes, my daughter, my mom & I all dressed as Minnie Mouse so we used red & white dotted ribbon to decorate around the front and then hung a bow from the canopy. For general Halloween theme, you could try wrapping the orange & black garland around the sides (depending on your stroller).
I've also used solar powered lights for Christmas and wrapped them around the sides & handle. It was cute, but honestly the lights were quite small & expensive (because it was solar powered). I'm not sure if anyone has better ideas for that.