So often the regular posters forget to mention simple basics about life and how to enjoy Budapest when giving advice. Perhaps those with knowledge and experience could add a few things here for a simple "How to do Budapest" check list for the newcomer? This could include what you HAVE to do and what you absolutely should NOT do (e.g: transit tickets). I know many of these things are covered in many different areas, but if we could put them all together in one place I think it would be helpful.
1. Don't ever plan to visit museums on a Monday, either in Budapest, or anywhere else in Hungary. They are all closed and you will then be trying to figure out how to fill your day. Monday is a TRAVEL DAY.