As for Halley's Comet, the surface would be a pitted surface, with craters and fault lines outlined by bright icy rays and heaps of rocks and dust piled up everywhere as the water ice that originally bound them into a lattice/conglomerate has evaporated. There would be many active and some inactive vents spewing jets of water vapor and gases into the Coma of the comet. There would also be a haze of vapor and frozen ices that would surround the nucleus and grow thicker as the comet gets nearer the Sun. Think of a 'dirty iceburg' covered with dust, gravel and exposed icy patches.