Whatever details are added to the sun, whether clouds or wisps, always paint some glare on top of them. I prefer to do this with an airbrush using a very saturated mid-tone on a Screen layer. This gives a better sense of shining light rather than just brightness and helps to unify whatever is painted beneath.
I don't think I have any suggestions for specific effects... after all CGI is basically just a way of adding things that can't just be photographed, for instance... so... your ideas need to come first. Maybe browse through some images of atmospheric effects to get some ideas for what could look cool in your own sun picture?