I’ve had my first few goes with this monster, and even in fairly average conditions it is pretty awesome!
This is a crappy iPhone shot through the eyepiece and it shows a huge prominence erupting from the solar surface. These are loops of plasma which arc along magnetic flux lines between sunspots. They can easily be multiple times larger than the earth, five to ten times the diameter, huge structures. The broad dark line across the surface is called a filament; this is basically a prominence but viewed against the hot, bright surface they are cooler, and appear darker.
They move at hundreds of thousands of miles an hour, and sometimes you can see them change in real time; this happened to me at the weekend, I watched a prominence arcing over then dropping little plasma bombs back to the surface. Quite amazing to see.