meowycrystal welcome to the QR forum. Many of your questions, including those as yet unasked, have already been answered. You can find many of those answers by using the dashboard. The Wikis are kept up to date so they are very reliable sources of information.

Unless you go to the markets, souks etc. I very much doubt that you will need more than your Credit card.

With regard to the Free City Tour look at timings etc. on http://www.qatarairways.com/dk/en/of...city-tour.page. Note especially the opening times for tour check-in. The first and last tours of the day are very popular and quickly become full.

Above all relax and enjoy the journey. You are in good hands.