If these billboards from Britain are coming to a bus near you, head for Ecclesiastes and preach your little heart out.
I'd start with expounding the text on the side of the bus ... then I'd go to the sympathetic approval of such a text in the early chapters of Ecclesiastes ... before going to those supreme chapters at the end - 11 & 12 - where the bus-text is slam-dunked and snookered, cornered and trumped.
And then live your life like Ecclesiastes 11 and 12 is true. Behind the "you do not knows" of chapter 11 (which encourage us to live adventuresome, risky lives) lies the God to be feared and obeyed of chapter 12. And that is the way to enjoy life...