The scaffolding was a stage erected for New Year’s Eve festivities. I assume the flowers and lights were part of that function as well, but my experience is limited; I only saw the Opera House before and after New Year’s Eve. (We were on a junk boat over the holiday itself.)
Tháp Rùa is the Turtle Tower in the middle of Hoàn Kiếm Lake, which is the “Lake of the Returned Sword” or–more simply–“Sword Lake.” The lake is in the heart of modern-day Hanoi, and is sacred in Vietnamese folklore.
As the story goes, a Turtle god surfaced on this lake to ask a King named Lê Lợi to give up his sword. The sword was called “Heaven’s Will” (i.e. Thuận Thiên) and was said to have been given to the King by a local deity called the Dragon King (i.e. Long Vương.) Apparently, the Vietnamese King took a “the gods giveth, and the gods taketh away stance,” turning the sword over to the Turtle deity.