Ok so the event's coming May 2nd. Unlike most of inno's past events the minigame in this one involves more than just choosing chests
You get an infinite dig that you have to make a tunnel through. Certain squares when cleared will contain vases (which are the chests for the event), idols (which gain you progress towards the grand prize), or more tools to keep digging longer. You primarily gain tools by buying them with scrolls which you gain from the questline or diamonds.
The grand prize: World's Fair, a 6x4 10 level residential building that at max level produces 10 FP and 10 goods when motivated.
New prize (available through daily specials): Carousel - it's 3x3, pretty and animated. It gives culture, attack bonus, and defense bonus. It can be thought of as an improved winner's plaza.
Otherwise daily specials will contain the usual variety of stuff.
Event planning tips:
Safe: Getting level 10 on the world's fair without diamonds will require completing the questline and skipping some amount of vases most likely. If your main goal is to complete it you can treat vases the same as empty sand until you're close enough to completing 10 grand prizes (200 idols total). You'll get some number of vases anyways.
Greedy: If you dig well it won't take too many diamonds to complete the level 10 grand prize while grabbing every vase along the way.
Diamond-Crazy: It'll take a great deal of diamonds to get a 2nd world's fair to level 10 (think 30-40k). If this is going to be your goal you'll want to skip vases the same as the safe player to minimise how many you have to spend.
General Strategy:
1) Buy 330 scroll tool packages. They are *MUCH* more efficient than any other way of buying tools. You may buy the occasional spare brush for 10 scrolls. All other options are big wastes.
2) Try to maximize the number of shadowed tiles revealed with your tool uses. When you see an idol in a direction you weren't planning to go you can either use a shovel if there's much to reveal vertically or move past it one tile further with a brush and then dynamite that square to keep making progress while grabbing it. Also try to balance your tool usage. If you're getting stuck with a lot of extra shovels or dynamite from the 330 pack look for more opportunities to use them.
3) Remember to click on and collect things you cleared the sand from.
4) You do not need to clear or even reveal every tile of sand. You cannot clear bricks.
5) Revealed tiles that don't appear to have anything hidden don't have anything hidden.
6) Empty tiles can either be used to progress faster to the right or when they've slid to the left anything next to them will be revealed anyways.
In terms of pacing, assuming you complete the questline and get every daily login you need to average 17 idols per 330 scroll package to get level 10 without diamonds. You can use this to guide whether you need to figure out ways to dig more efficiently or to skip more vases that are out of the way.