MasyaAllah, what can one say about the greatest masjid on earth? A marvelous and spiritual experience and the sight of the Kaabah is truly special. Be patient with the crowds and remember Allah at all times. Be careful with your shoes / slippers as it might disappear. Bring them with you using the plastic bags available. If you lose them during the hot day, be careful of the hot floor surface which are not marble. If you are lucky you might find someone selling slippers.
Try to make effort to pray jemaah in the masjid at all times when you are here.
May Allah preserve and protect this great masjid and its access for all mankind n Muslims.