Another kudos point to our riddle master, Cocomino, on this one.
花火 (はなび, fireworks)
This one is more straightforward than some of our recent word plays. The question asks "What kind of navigation system do you use on clear, summer days?" The trick here is that 「ナビ」 (navigation system) is also part of the reading for the word 「はなび」, "fireworks." So it's not a navigation system at all.